An error in a message handler causes all message handlers to fail

[zuzak]

You can have multiple event handlers listening for a message.
This means that each message can have multiple things happen at once.

However, it seems like if one of those executed functions throws an error, it prevents execution of the other message functions. For example, if the scrabble module throws an error (#281), the URL shortener won't kick in (#277).

The bot should handle errors properly, so that an error being thrown in one event handler does not cause the other event handlers to fail.