Add 'handle_exception' coderef attr

The idea here is to provide more granular control over how thrown
exceptions (e.g., from HTTP::Throwable) are handled.

One specific example: a user could provide a custom handle_exception
hook that lets the thrown exception through in a development
environment, while intercepting it and returning a more innocuous
error (sans stack trace, etc.) in a production environment.
Update tests for handle_exception (#2)
update changes too.
Merge remote-tracking branch 'upstream/master' into handle_exception

Conflicts:
	Changes
Merge pull request #2 from genehack/handle_exception

Add 'handle_exception' coderef attr