Follow-up from "Streamline execute logic for Projects::UpdateMirrorService"

The following discussion from !162678 (merged) should be addressed:

• @brodock started a discussion: (+1 comment)

  Should we consider refactoring to use a ServiceResponse instead of using exceptions?

  We have this new pattern as the preferred way to report failure or success: https://docs.gitlab.com/ee/development/reusing_abstractions.html#serviceresponse