Cellworx: fix off-by-one if all pixels files are missing

This won't allow .htd files without TIFFs to import, but it makes the
exception that is thrown more informative.

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/6332.  The QA dataset mentioned in the ticket should throw a ```FileNotFoundException``` with this PR, as all of the pixels files are missing from the dataset.