handle unavailable compression methods in mysqldump plugin
handle compression errors in mysql-lvm
handle holland compression errors in xtrabackup
fix formatting for exceptions
change check_available_space() to accept a spool_entry
check spool_entry.path rather than spool.path

This handles cases where individual backupsets may be symlinked.
implement base method for freeing old backups before starting a new backup
purge old backups on demand

This finalizes purging old backups when there is insufficient space
for a new backup.
add [holland:backup] -> purge-on-demand option

This option makes holland backup attempt to purge old backups
rather than aborting a backup due to lack of free space.
update changelog
improve logging for purge-on-demand