Use Display instead of Debug in the default error handler (#18629)

# Objective

Improve error messages for missing resources.  

The default error handler currently prints the `Debug` representation of
the error type instead of `Display`. Most error types use
`#[derive(Debug)]`, resulting in a dump of the structure, but will have
a user-friendly message for `Display`.

Follow-up to #18593

## Solution

Change the default error handler to use `Display` instead of `Debug`.  

Change `BevyError` to include the backtrace in the `Display` format in
addition to `Debug` so that it is still included.

## Showcase

Before: 

```
Encountered an error in system `system_name`: SystemParamValidationError { skipped: false, message: "Resource does not exist", param: "bevy_ecs::change_detection::Res<app_name::ResourceType>" }

Encountered an error in system `other_system_name`: "String message with\nmultiple lines."
```

After

```
Encountered an error in system `system_name`: Parameter `Res<ResourceType>` failed validation: Resource does not exist

Encountered an error in system `other_system_name`: String message with
multiple lines.
```

crates/bevy_ecs/src/error/bevy_error.rs

@@ -34,48 +34,11 @@ impl BevyError {
     pub fn downcast_ref<E: Error + 'static>(&self) -> Option<&E> {
         self.inner.error.downcast_ref::<E>()
     }
-}
 
-/// This type exists (rather than having a `BevyError(Box<dyn InnerBevyError)`) to make [`BevyError`] use a "thin pointer" instead of
-/// a "fat pointer", which reduces the size of our Result by a usize. This does introduce an extra indirection, but error handling is a "cold path".
-/// We don't need to optimize it to that degree.
-/// PERF: We could probably have the best of both worlds with a "custom vtable" impl, but thats not a huge priority right now and the code simplicity
-/// of the current impl is nice.
-struct InnerBevyError {
-    error: Box<dyn Error + Send + Sync + 'static>,
-    #[cfg(feature = "backtrace")]
-    backtrace: std::backtrace::Backtrace,
-}
-
-// NOTE: writing the impl this way gives us From<&str> ... nice!
-impl<E> From<E> for BevyError
-where
-    Box<dyn Error + Send + Sync + 'static>: From<E>,
-{
-    #[cold]
-    fn from(error: E) -> Self {
-        BevyError {
-            inner: Box::new(InnerBevyError {
-                error: error.into(),
-                #[cfg(feature = "backtrace")]
-                backtrace: std::backtrace::Backtrace::capture(),
-            }),
-        }
-    }
-}
-
-impl Display for BevyError {
-    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
-        writeln!(f, "{}", self.inner.error)?;
-        Ok(())
-    }
-}
-
-impl Debug for BevyError {
-    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
-        writeln!(f, "{:?}", self.inner.error)?;
+    fn format_backtrace(&self, _f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         #[cfg(feature = "backtrace")]
         {
+            let f = _f;
             let backtrace = &self.inner.backtrace;
             if let std::backtrace::BacktraceStatus::Captured = backtrace.status() {
                 let full_backtrace = std::env::var("BEVY_BACKTRACE").is_ok_and(|val| val == "full");
@@ -123,7 +86,50 @@ impl Debug for BevyError {
                 }
             }
         }
+        Ok(())
+    }
+}
 
+/// This type exists (rather than having a `BevyError(Box<dyn InnerBevyError)`) to make [`BevyError`] use a "thin pointer" instead of
+/// a "fat pointer", which reduces the size of our Result by a usize. This does introduce an extra indirection, but error handling is a "cold path".
+/// We don't need to optimize it to that degree.
+/// PERF: We could probably have the best of both worlds with a "custom vtable" impl, but thats not a huge priority right now and the code simplicity
+/// of the current impl is nice.
+struct InnerBevyError {
+    error: Box<dyn Error + Send + Sync + 'static>,
+    #[cfg(feature = "backtrace")]
+    backtrace: std::backtrace::Backtrace,
+}
+
+// NOTE: writing the impl this way gives us From<&str> ... nice!
+impl<E> From<E> for BevyError
+where
+    Box<dyn Error + Send + Sync + 'static>: From<E>,
+{
+    #[cold]
+    fn from(error: E) -> Self {
+        BevyError {
+            inner: Box::new(InnerBevyError {
+                error: error.into(),
+                #[cfg(feature = "backtrace")]
+                backtrace: std::backtrace::Backtrace::capture(),
+            }),
+        }
+    }
+}
+
+impl Display for BevyError {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        writeln!(f, "{}", self.inner.error)?;
+        self.format_backtrace(f)?;
+        Ok(())
+    }
+}
+
+impl Debug for BevyError {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        writeln!(f, "{:?}", self.inner.error)?;
+        self.format_backtrace(f)?;
         Ok(())
     }
 }

crates/bevy_ecs/src/error/handler.rs

@@ -127,7 +127,7 @@ pub fn default_error_handler() -> fn(BevyError, ErrorContext) {
 macro_rules! inner {
     ($call:path, $e:ident, $c:ident) => {
         $call!(
-            "Encountered an error in {} `{}`: {:?}",
+            "Encountered an error in {} `{}`: {}",
             $c.kind(),
             $c.name(),
             $e

crates/bevy_ecs/src/system/system_param.rs

@@ -2932,7 +2932,7 @@ mod tests {
     }
 
     #[test]
-    #[should_panic = "Encountered an error in system `bevy_ecs::system::system_param::tests::missing_resource_error::res_system`: SystemParamValidationError { skipped: false, message: \"Resource does not exist\", param: \"bevy_ecs::change_detection::Res<bevy_ecs::system::system_param::tests::missing_resource_error::MissingResource>\" }"]
+    #[should_panic = "Encountered an error in system `bevy_ecs::system::system_param::tests::missing_resource_error::res_system`: Parameter `Res<MissingResource>` failed validation: Resource does not exist"]
     fn missing_resource_error() {
         #[derive(Resource)]
         pub struct MissingResource;

examples/ecs/error_handling.rs

@@ -55,7 +55,7 @@ fn main() {
 
     // If we run the app, we'll see the following output at startup:
     //
-    //  WARN Encountered an error in system `fallible_systems::failing_system`: "Resource not initialized"
+    //  WARN Encountered an error in system `fallible_systems::failing_system`: Resource not initialized
     // ERROR fallible_systems::failing_system failed: Resource not initialized
     //  INFO captured error: Resource not initialized
     app.run();