# Objective
Improve the parameter validation error message for
`Event(Reader|Writer|Mutator)`.
System parameters defined using `#[derive(SystemParam)]`, including the
parameters for events, currently propagate the validation errors from
their subparameters. The error includes the type of the failing
parameter, so the resulting error includes the type of the failing
subparameter instead of the derived parameter.
In particular, `EventReader<T>` will report an error from a
`Res<Events<T>>`, even though the user has no parameter of that type!
This is a follow-up to #18593.
## Solution
Have `#[derive]`d system parameters map errors during propagation so
that they report the outer parameter type.
To continue to provide context, add a field to
`SystemParamValidationError` that identifies the subparameter by name,
and is empty for non-`#[derive]`d parameters.
Allow them to override the failure message for individual parameters.
Use this to convert "Resource does not exist" to "Event not initialized"
for `Event(Reader|Writer|Mutator)`.
## Showcase
The validation error for a `EventReader<SomeEvent>` parameter when
`add_event` has not been called changes from:
Before:
```
Parameter `Res<Events<SomeEvent>>` failed validation: Resource does not exist
```
After
```
Parameter `EventReader<SomeEvent>::events` failed validation: Event not initialized
```
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