Parse registration field errors

[trex]

Registration requires that the username, email, and password fields are all present. The authentication server also performs validation against username and email uniqueness, with the option to configure password validation rules. If these fields fail validation the authentication server returns errors.

We need to relay these to the end user so they can fix the issue on the registration form.

[kmid5280]

Currently if you try to register a new user using a user name or email that already exists, the error will show up in the console on the back end with the specific issue. It will say for instance, "A User with email = [existing email] already exists." On the front end, you will only see a generic "something went wrong" message.

[trex]

This is related to #60