Include specific details about why the username is invalid
Summary: Inspired by T16386
Test Plan:
For each of the following flows, enter various invalid usernames and see sensible output:
- https://phorge.localhost/auth/register
- The "rename user" workflow in http://phorge.localhost/people/manage/1/
- http://phorge.localhost/people/new/standard/
The `PhabricatorUserEditor.php` class change can't be tested - it's a low-level failsafe that should be unreachable because higher-level code should validate usernames.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno, aklapper
Differential Revision: https://we.phorge.it/D26567