In phocube, the error thrown when the input is a projected image was...

In phocube, the error thrown when the input is a projected image was misleading. It has been fixed to be more helpful for the user. Fixes #923.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@5594 41f8697f-d340-4b68-9986-7bafba869bb8