push: indicate partialness of error message

push: indicate partialness of error message

The existing message indicates that an error occured during
push, but it is unclear whether _any_ refs were actually
pushed (even though the status table above shows which were
pushed successfully and which were not, the message "failed
to push" implies a total failure). By indicating that "some
refs" failed, we hopefully indicate to the user that the
table above contains the details.

We could also put in an explicit "see above for details"
message, but it seemed to clutter the output quite a bit
(both on a line of its own, or at the end of the error line,
which inevitably wraps).

This could also be made more fancy if the transport
mechanism passed back more details on how many refs
succeeded and failed:

  error: failed to push %d out of %d refs to '%s'

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>