Improve User username error for namespace conflict

What does this MR do?

Closes #215309

Overwrites the "has already been taken" error on Username when it comes from the 'namespace.path' existing but no User with that username.

before: Username has already been taken

after: A user, alias, or group already exists with that username.

Screenshots (strongly suggested)

before	after

• does not affect the reserved name check

before	after

• still returns "Username is already taken" when namespace is taken by a User username, (not a group or project namespace)

• doesn't remove other validations on specific fields

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #215309