Handle invalid masked setups more gracefully.

If the mask widget was not set up correctly, throw an Error with a hint.
Added test for [PR 370](https://github.com/angular-ui/angular-ui/pull/370)
Merge branch 'master' of git://github.com/sja/angular-ui

* 'master' of git://github.com/sja/angular-ui:
  Added test for [PR 370](https://github.com/angular-ui/angular-ui/pull/370)
  Handle invalid masked setups more gracefully.
chore(mask): Made petes suggested changes to #370