filter-branch: fail gracefully when a filter fails

filter-branch: fail gracefully when a filter fails

A common mistake is to provide a filter which fails unwantedly. For
example, this will stop in the middle:

	git filter-branch --env-filter '
		test $GIT_COMMITTER_EMAIL = xyz &&
		export GIT_COMMITTER_EMAIL = abc' rewritten

When $GIT_COMMITTER_EMAIL is not "xyz", the test fails, and consequently
the whole filter has a non-zero exit status. However, as demonstrated
in this example, filter-branch would just stop, and the user would be
none the wiser.

Also, a failing msg-filter would not have been caught, as was the
case with one of the tests.

This patch fixes both issues, by paying attention to the exit status
of msg-filter, and by saying what failed before exiting.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>