--no-cleanup fails with no message if *-artefact branch already exists

[simesy]

To reproduce you can have a master-artefact branch in your repo, and try deploying with the --no-cleanup flag. Robo fails with an empty message, the message is not getting picked up properly in the gitcommand component.

[simesy]

It appears to have caught me because I accidentally ran the artefact command on my repo on a build which had already run the command (that failed). I think the missing error is the bigger problem.