Added error checks to several fread(...) and fgets(...) calls due to the...

Added error checks to several fread(...) and fgets(...) calls due to the compiler warnings that cropped up on prog3 because return values were being ignored.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@2648 41f8697f-d340-4b68-9986-7bafba869bb8