Descriptive errors for GitLab token validation

When importing a project from GitLab:

The current personal access token validation is binary: success or failure.
While the http response contains a more descriptive error message, this cannot be used by the UI to provide useful information to the user.

To improve this, the check_pat endpoint should return a 200 status code, with a descriptive status for the PAT:

• Valid
• Invalid
• Revoked
• Insufficient Scope

The UI should display an appropriate message to the user, following this status.