Updated Cnetedit to throw error message if a nonexistent network file is...

Updated Cnetedit to throw error message if a nonexistent network file is entered. Message tells user the file doesn't exist. Fixes #4305

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@7161 41f8697f-d340-4b68-9986-7bafba869bb8