Deprecate :after-handler value for show_exceptions

It's very confusing for new users as they will write custom handlers
with `error(*codes, &:block)` but those would never run in development
because `handle_exception!` was raising a general exception before
checking for custom errors.

I believe the only advantage for that would be to display stack traces
on the browser for users but I think most of the times users are catching
custom errors they don't care much about the stack trace or even if they
care they can always raise the exception again