Look for dodgy characters in filenames

[Chris Warburton]

In particular, I came across some dodgy apostrophes (possibly Microsoft
bollocks?) in some Saxon filenames.

This prevented Picard from loading the directory contents; even worse,
it didn't even show a warning, it just ignored files which were accessed
*after* the first dodgy one.

We can be easily misled that we're tagging everything, when in fact an
arbitrary number of files may be getting skipped entirely by Picard.