Allow fallback error handlers to queue commands

[0xEgao]

Objective

Fixes #24821.

Solution

• Pass Commands to ErrorHandler and return it from the handler.
• Apply commands queued by error handlers across executors, observers, combinators, and command error paths.
• Preserve existing scheduling behavior when no error occurs.
• Add a set_error_handler doc example demonstrating how to send AppExit.

[alice-i-cecile]

I really want a feature like this, but this might have surprising performance implications. Can you please benchmark this?

[0xEgao]

I really want a feature like this, but this might have surprising performance implications. Can you please benchmark this?

Yeah sure :)