IException.h: 2012-03-13 Steven Lambright - toString() was giving "End" with...

IException.h: 2012-03-13 Steven Lambright - toString() was giving "End" with an empty exception when using Format = Pvl. It should (and must) be an empty string. This is fixed -- fixes #755. 

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@4115 41f8697f-d340-4b68-9986-7bafba869bb8