Fix `rake gitlab:incoming_email:check`

What does this MR do?

Fix rake gitlab:incoming_email:check and make it report error properly, so that we know what's really wrong.

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

• I think it's fine to show the error message?
• Do we have tests? I guess not...

Why was this MR needed?

Currently it's broken because config was never defined, and it would be rescued and always report "no".

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

What are the relevant issue numbers?

Closes #38197 (closed)