do not override receive-pack errors

do not override receive-pack errors

Receive runs rev-list --verify-objects in order to detect missing
objects. However, such errors are ignored and overridden later.
Instead, consequently ignore all update commands for which an error has
already been detected.

Some tests in t5504 are obsoleted by this change, because invalid
objects are detected even if fsck is not enabled. Instead, they now test
for different error messages depending on whether or not fsck is turned
on. A better fix would be to force a corruption that will be detected by
fsck but not by rev-list.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>