Wnen refusing to push a head, we said cryptic "remote 'branch'
object X does not exist on local" or "remote ref 'branch' is not
a strict subset of local ref 'branch'". That was gittish.
Since the most likely reason this happens is because the pushed
head was not up-to-date, clarify the error message to say that
straight, and suggest pulling first.
First noticed by Johannes and seconded by Andreas.