Improve the panic message for schedule build errors (#7860)

# Objective

The `ScheduleBuildError` type has a `Display` implementation which beautifully formats the error. However, schedule build errors are currently reported using `unwrap()`, which uses the `Debug` implementation and makes the error message look unfished.

## Solution

Use `unwrap_or_else` so we can customize the formatting of the error message.

crates/bevy_ecs/src/schedule/schedule.rs

@@ -245,7 +245,7 @@ impl Schedule {
     /// Runs all systems in this schedule on the `world`, using its current execution strategy.
     pub fn run(&mut self, world: &mut World) {
         world.check_change_ticks();
-        self.initialize(world).unwrap();
+        self.initialize(world).unwrap_or_else(|e| panic!("{e}"));
         self.executor.run(&mut self.executable, world);
     }