Swallow errors in ReactiveLabeler when label doesn't exist
requested to merge 981-gitlab-error-badrequest-server-responded-with-code-400-message-400-bad-request-note-note-can-t into master
When the author ask to set a label that doesn't exist, we shouldn't raise a Sentry exception as it's a client error.
We could also display a message to the author to tell them that the label doesn't exist?
Closes #981 (closed).
Edited by Rémy Coutable