Clean up the Boku form when there are errors (bug 1013434)

https://bugzilla.mozilla.org/show_bug.cgi?id=1013434

Before
![Before](https://cloud.githubusercontent.com/assets/211578/3307268/6fffe122-f66a-11e3-8008-7845d5003c6b.png)

After
![After](https://cloud.githubusercontent.com/assets/211578/3307294/a8cc16b0-f66a-11e3-8a1d-559f515d4ef5.png)