Allow `FallbackErrorHandler` to send `AppExit`

## What problem does this solve or what need does it fill?

Cannot exit app without panic when error occurs.

I'm developing game server and need to keep server running.

## What solution would you like?

```rust
pub type ErrorHandler = fn(BevyError, ErrorContext); /// change to-> pub type ErrorHandler = fn(BevyError, ErrorContext, MessageWriter);

#[derive(Resource, Deref, DerefMut, Copy, Clone)]
pub struct FallbackErrorHandler(pub ErrorHandler);
```

## What alternative(s) have you considered?

```rust
pub type ErrorHandler = fn(BevyError, ErrorContext); /// change to-> pub type ErrorHandler = fn(BevyError, ErrorContext) -> Option;

#[derive(Resource, Deref, DerefMut, Copy, Clone)]
pub struct FallbackErrorHandler(pub ErrorHandler);
```

Locate FallbackErrorHandler, ErrorHandler, BevyError, ErrorContext, and AppExit in the repository, then read the surrounding error-handling and application-exit code. Compare the requested callback signatures and determine how AppExit should be delivered without forcing a panic; done means the fallback handler supports the requested exit behavior while allowing the game server to keep running when appropriate.

Written by the indexing model from the issue text.