Prevented cneteditor from crashing when attempting to open nonexistent files...

Prevented cneteditor from crashing when attempting to open nonexistent files and added accompanying error messages. Fixes #2363

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@6887 41f8697f-d340-4b68-9986-7bafba869bb8