need a better error message when mediator isn't actually extending AbstractMediator or implementing the interface provideDecoratedElement

[jonbcampos]

See this link:
http://stackoverflow.com/questions/16307116/viewstack-selected-seems-to-work-every-other-time

The issue was that I stubbed out the mediators but didn't actually extend or implement. I had guessed that the mediator wouldn't work but not that a serious of unfortunate events would follow. Once fixed everything worked as expected. Could use a better error message and we should be good.