Make cvmfs_transaction script more robust
I had a manual transaction fail to acquire the lock due to:
Waiting for existing transaction to complete, this might take a while...
..............lhcb-test.cern.ch is in a transaction but /cvmfs/lhcb-test.cern.ch is not mounted read/write
WARNING: The repository lhcb-test.cern.ch is currently publishing and should not
be touched. If you are absolutely sure, that this is _not_ the case,
please run the following command and retry:
rm -fR /var/spool/cvmfs/lhcb-test.cern.ch/is_publishing.lock