Improved logging when something fails

This is kinda important.

I'm not sure where the best place is to put this? Should we add a new plugin ala Notify.php, just amend the Notify plugin like this, or add the change directly to BackupMigrate::backup()?

@DamienMcKenna amending the Notify plugin works for me, I applied the patch successfully on 5.1.x, not sure how to "force" the backup to fail in order to test this, any suggestions?