bodhi-push will happily resume Composes that are not in a stopped state

[bowlofeggs]

bodhi-push does not prevent or warn users when they ask it to resume a compose that isn't in a stopped state (which I think would only include the failed state). This is bad because the backend will happily run two simultaneous composes if asked to do so.

We should make it error if the compose isn't in a stopped state, or at least warn. I lean towards erroring and refusing, but there are situations (such as fedmsg-hub stopping) where composes could get left in what appears to be a running state even when they are not running, and it would be nice if there were a way to force it to start over.