Fix error message display in event registration modal.

#5392 

Followup to https://github.com/openscholar/openscholar/issues/5392#issuecomment-38180960

After the fix:
![selection_032](https://f.cloud.github.com/assets/4497748/2509619/e474fe4e-b3f7-11e3-8eb1-69fd48de7892.png)