Changing an account email address to one that already exists returns a 500 error.

[hodgestar]

The following error is thrown when attempting to use the emailed token on a URL like https://go.vumi.org/t/task/?token=5-dbcedf:

File "/var/praekelt/vumi-go/go/token/views.py", line 66, in token_task 
    callback(*callback_args, **callback_kwargs) 
File "/var/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/celery/app/task.py", line 330, in __call__ 
    return self.run(*args, **kwargs) 
File "/var/praekelt/vumi-go/go/account/tasks.py", line 22, in update_account_details 
    user.save() 
... 
IntegrityError: duplicate key value violates unique constraint "base_gouser_email_key". 
DETAIL: Key (email)=([email protected]) already exists.

We should give the user clearer feedback and not return a 500 error.