Dave Shoreman d18a0a6453 Add missing ApplicationException dependency
The relation backend formwidget throws an `ApplicationException` when a relation isn't defined, but the exception is missing its `use` statement. This PR adds it in. See error below:

![Screenshot showing error thrown](http://i.imgur.com/4Ka6fL2.png)
2015-04-01 19:13:42 +01:00
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