Add user-friendly message upon runner registration limit

What does this MR do?

This MR is a follow-up to !60157 (merged) in order to improve the error messages shown to the user in case of running against the limit when registering a runner.

Screenshots (strongly suggested)

Current state:

Desired state:

• Command line:

  pry(main)> Plan.default.actual_limits.update!(ci_registered_project_runners: 1)

  $ curl --request POST --header "PRIVATE-TOKEN: ${TOKEN}" "http://localhost:3000/api/v4/projects/21/runners" --form "runner_id=24"
  {"message":{"assign_to":["Validation failed: Maximum number of ci registered project runners (1) exceeded"]}}

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because it is behind a feature flag.
• Documentation (if required)
• Code review guidelines
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• Style guides
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• 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

Closes #321368