Added support for warning about and skipping files.

Files that do not exist or user does not have read access to
are skipped and users are warned that the file is skipped.
I think this should either be renamed or have an accompanying doc string (or both).  Part of the problem of having a boolean function that also has a side effect (in this case, queueing entries on a result queue) is that it's not obvious that you're not suppose to call this function more than once on a path now.  Aside from restructuring code, I'd at least make it obvious not to call this twice on the same file.