Better code around the message display on signup.

 * adds the missing default message display that the page class expects.
 * makes the code smarter about looking up message display by class, not id so subclasses don't break
 * fix relocate check.

This prevents SwatWidgetNotFoundExceptions on live sites, such as https://stage2.silverorange.com/exceptions/essentials/2014-12-10/exception-8982a6a39be2946dcec44c3bf7889032.html

These messages happen when entered a banned address, such as `test@test.com`
Merge pull request #13 from isaacgrant/missing-message-display-errors

Better code around the message display on signup.