Better error reporting on failed signups with multiple problems.

Currently, if signup fails, we only give back one error message of the following:

- Username taken
- Failed Captcha
- Password to common / short
- Password confirmation didn't match

@patrick-mobi pointed out that it would be ideal to have feedback on _all_ sources of problems instead of just one ("This username is already taken" next to the username field and "The password confirmation didn't match with the password" next to that text field, etc.). That would involve a substantial rework. We still might want to do that eventually.