v0.19: Improved error reporting/handling

* Working on improving error handling in the file object.  This is not complete, but works well enough to find a few errors that have been causing us problems (notably, find is occasionally failing building the archive async manifest when system is under load).

* Found and squashed a nasty bug where file_copy was defaulted to ignore errors.  There was also an issue in file_exists that was causing the test to fail when the file actually did exist.  Together they could have resulted in a corrupt backup with no errors, though it is very unlikely.