gc: always clean up the packwriter; abort on errors

gc: always clean up the packwriter; abort on errors

Make sure to always close or abort the packwriter.  Abort on errors
under the assumption that any packfile in progress should only contain
blobs duplicated from the packfiles that were being garbage collected
to produce it, and those packfiles should never be removed until we've
finished writing and closing the one they're being swept into.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>