check for file existence during "Load Model" (including error dialog)
 * fixed bug: http://sourceforge.net/tracker/index.php?func=detail&aid=2973875&group_id=237831&atid=1104176


git-svn-id: https://pycam.svn.sourceforge.net/svnroot/pycam/trunk@263 bbaffbd6-741e-11dd-a85d-61de82d9cad9