mysqldump-lvm does not log enough on failure

When a snapshot is invalidated and mysqld crashes as a result, there is no information logged about what went wrong. Only by correlating dmesg or other logging to the time when holland failed is it all clear that the reason a backup failed was due to snapshot invalidation (e.g. ran out of space allocated to the snapshot)

holland lvm plugins really should always check the snapshot volume at the end and if lv_attr has an 'S' flag, we should verbosely log that the snapshot appears to have been invalidated.

Further the mysqldump-lvm plugin should try to grab the last few lines of the mysqld error log to indicate what failed, if holland's spawned mysqld process is not running when we attempt to terminate it.