pre-receive/update: squelch harmless error message
If install.sh (e.g. make install) is run while a simultaneous receive or
mirror update is in progress, list_packs may be temporarily unavailable.
This is harmless since the worst that could happen is that the
.needsgc file will not be created until one extra pack is fetched/received.
Therefore squelch any error/complaint/failure from a missing list_packs
executable in both the pre-receive and update.sh scripts.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>