Make username update fail if namespace update fails

What does this MR do?

Helps preserve integrity of user username == namespace name/path. Does not allow username renames to save if the namespace name and path renames fail (e.g. if a group exists with that name).

Why was this MR needed?

https://gitlab.com/gitlab-com/support-forum/issues/2316

Screenshots (if relevant)

(Instead of success message, and having the username diverge from the namespace name/path)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-com/support-forum/issues/2316