Zeiss TIFF: fix used file list when expected files are missing

The top-level .tif file in particular can be missing without causing any
other problems.  This prevents any files that do not exist from
appearing on the used file list, as that would cause import into OMERO
to fail.
Zeiss TIFF: fix one more bug when the master .tif file is missing

This prevents an exception from being thrown if the master .tif file is
missing when the recorded original name ends with .zvi.
Merge pull request #1422 from melissalinkert/zeiss-tiff-missing-file

Zeiss TIFF: fix used file list when expected files are missing