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.
Signed-off-by: NJunio C Hamano <junkio@cox.net>