Coerce return value

Duplicity returns quite random, undocumented return values.
Also the pre-commands will might do that.

Since ```duplicity::monitored_job``` should only report 0, 1 or 2 to the periodicnoise component, let's coerce 
all non-zero return values into critical monitoring event. 

Explicitly NOT made optional, as I cannot think of any non-critical backup. If a backup is not critical to have, 
it should better not be done at all.