feat(client): Allow the user to restart the signup flow on email bounce.

feat(client): Allow the user to restart the signup flow on email bounce.

Add two new error codes:
* 1018 - SIGNUP_EMAIL_BOUNCE
* 1019 - DIFFERENT_EMAIL_REQUIRED

fixes #1801
> To clarify, you can load the settings page in a new tab (where Session.uid would be undefined) then try to change your password on an unverified account.

I was wondering how to get to this. With your `User` model PR, unverified users that visit the settings are immediately sent to the `/confirm` page as well, correct?