git remote update: Report error for non-existing groups
git remote update: Report error for non-existing groups

Previosly, git remote update <non-existing-group> would just silently fail
and do nothing. Now it will report an error saying that the group does
not exist.

Signed-off-by: Finn Arne Gangstad <919d28b79e74000ff06c8c9bca64059d5449989b@pvv.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git