`cta-tape-label` should have clearer errors

This is the log output we were seeing cta-tape-label on the failing CI jobs:

Oct  7 13:55:30.095091 tpsrv01 cta-tape-label: LVL="ERROR" PID="347" TID="347" MSG="Label session failed to label the tape" userName="UNKNOWN" tapeVid="V06001" tapeOldLabel="" force="false" tapeLoadTimeout="7200" tapeLabelError="[TapeLabelCmd::checkTapeLabel] - Reading VOL1: Failed ST read in DriveGeneric::readExactBlock Errno=12: Cannot allocate memory"

As discussed in today's dev meeting this message is not so clear for newcomers: we were looking for possible SCSI errors.