When an invalid file is entered thmproc will display an error message instead...

When an invalid file is entered thmproc will display an error message instead of crashing. Fixes #1025.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6271 41f8697f-d340-4b68-9986-7bafba869bb8