Handle FAM monitor failures more gracefully:

* Where possible, create the file or directory that is about to be
  monitored.  This ensures that content can be added later without
  need to restart Bcfg2.  (Otherwise, adding the monitor would fail,
  and so when you did create the file in question, bcfg2-server would
  never be notified of it.)
* When not possible, give better error messages.