Delete fails if remote doesn't exist

[dermojo]

I have configured deleteRemote = true in my gitconfig for various branch types.
When working with multiple people on the same branch or when merging via web UI (we're using GitLab), I have to delete the local topic branch afterwards. A normal "git flow delete" fails because it tries to delete the remote, which doesn't exist.

Suggested fix: check if the remote branch exists before trying to delete it.