exit instead of return false on image failure

pandatool/src/imagebase/imageReader.cxx

@@ -48,7 +48,7 @@ handle_args(ProgramBase::Args &args) {
 
   if (!_image.read(args[0])) {
     nout << "Unable to read image file " << args[0] << ".\n";
-    return false;
+    exit(1);
   }
 
   return true;