Gracefully report registration email conflict

[gregelin]

Describe the bug as a Story Registering a new user with same username as existing causes a crash instead of graceful error message.

To Reproduce Steps to reproduce the behavior:

1. Logout
2. Register account with 'newusername'
3. Logout
4. Register account with 'newusername'
5. Crash!

Expected behavior Report a message that tell user the chosen username is not available.

Screenshots

[ward438]

@gregelin

• Could not reproduce error. Attempted to register new user to produce error regarding duplicate users. Registered new user with pre-existing username without error. Was prompted with 'A user with that username already exists.'