Better messaging

[jamezpolley]

At the moment most error conditions just result in a redirect back to / or /events with no feedback about what happened.. Do something nicer - having an div in base.html would be nice, and then we could just pass an "error" value along to any template.

Similarly, succeses - such as when a user succesfully submits a talk offer - usually just redirect somewhere random, or show the user the form again.

A generic messaging framework could display both error and success messages to the user.