fail mysqldump backup if no databases were discovered
Merge branch 'lp_1204208'
update CHANGES.txt about mysqldump change for lp#1204208
cleanup logging for purge-on-demand

previously the logging only logged the purged space not the
total free, which was confusing
be explicit about space available with and after purge-on-demand

logging was vague about the total free space after a purge operation
and was not clear about the total free space prior to starting the
backup.  Now the purged space and total free space is noted
and after purging the total free space on the target filesystem is
noted.