Updates the GitLab client to return an error when a domain is not found. Handle the error returned by the Resolver when loading a domain. This allows us to report errors we encounter between Pages and GitLab rails instead of falling back to disk serving and failing silently.
It also modifies the mock server we use for acceptance tests so that we can expand our acceptance tests when using the API as source. Related to #470.
Closes #494 (closed)