[MERGE] Fix for 306394: warning, not error, on non-existent file.

[Stephen J. Turnbull]

Aaron Bentley writes:

 > I think that the editor-cum-MUA should leave *more* of the file status
 > tracking to Bazaar, so that not only does it not duplicate the effort,
 > it can't cause the error.

That's a nice theory, but in the real world we have globs and other
algorithmic ways of constructing sets of filenames.  It *can* cause
the error.