Modified setScale method to allow users to zoom in closer and to prevent...

Modified setScale method to allow users to zoom in closer and to prevent zooming out too far.  Added iExceptions to ViewportBuffer help track errors.

git-svn-id: http://subversion.wr.usgs.gov/repos/prog/isis3/trunk@152 41f8697f-d340-4b68-9986-7bafba869bb8