gpg: do show gpg's error message upon failure
There are few things more frustrating when signing a commit fails than
reading a terse "error: gpg failed to sign the data" message followed by
the unsurprising "fatal: failed to write commit object" message.
In many cases where signing a commit or tag fails, `gpg` actually said
something helpful, on its stderr, and Git even consumed that, but then
keeps mum about it.
Teach Git to stop withholding that rather important information.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>