Resolve "Bug: When creating an account with invalid characters the error is "Username already taken" not "Invalid characters used""

What does this MR do?

Fixes the display of username validation error messages in the signup form.

Are there points in the code the reviewer needs to double check?

No

Why was this MR needed?

Because of #45575 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?

Closes #45575 (closed)