remove error dialog for account exists /mobile create account

This PR addresses https://optimizely.atlassian.net/browse/MKT-780

To test, run the server and go to /mobile.  Try to create an account using `david.fox-powell@optimizely.com`.  You should get an account exists warning but no popup dialog

![screen shot 2014-11-18 at 3 07 49 pm](https://cloud.githubusercontent.com/assets/4656726/5097972/c5460118-6f35-11e4-8daa-a9ca1bc94e2e.png)

Now try to create an account with any other valid email and it should be successful and redirect.