Add error checking to prevent crashing when the ground source has the same...

Add error checking to prevent crashing when the ground source has the same serial number as a cube in the cube list file.  Fixes #1018. Fixes #1836

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4796 41f8697f-d340-4b68-9986-7bafba869bb8