Allow error events to be handled in Handlers

[CraZySacX]

Currently error events just spit out the error message. This allows the handler to react to error events outside of readable and writable.

[carllerche]

Ah yes... I completely forgot about this. I've been meaning to fix it :P

I'm wondering if it would be better to pass errors to the handler via readable and set the ReadHint -> error. The issue is that, I do not believe that every platform reliably detects errors without reading, so this may be a good case of just considering an error a "readable" event.

[CraZySacX]

Yeah that sounds better. I'll close this pull request and submit another.