fix(gitlab-reviewer): change from error to warning (#1803)

GitLab as of v10.6 has a API end point for Merge Request Approvals
which equates to the Reviewer functionality in GitHub.  This is available
to both GitLab.com and EES or above on-prem users.

See https://docs.gitlab.com/ee/api/merge_request_approvals.html#merge-request-level-approvals



Signed-off-by: Adam Moss <adam.moss@bcs.org.uk>