Support adding additional error handling to generated actors

[benbenwilde]

Description

Currently, if you want to log whenever a generated actor throws an error while handling a message, you have to override the long form of every message handler. This adds an OnError virtual method so that you can easily add logging or any other behavior in one place.

This change maintains exact behavior from before, as the default implementation of OnError does the same thing that the passed in onError callback always does. Even though the onError parameter is now technically redundant, it is retained to keep behavior of existing overrides unchanged.

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)