Throw an exception instead of returning an invalid value

If the code path should never be reached, having an exception thrown
makes it easier to nitce it was reached after all.

dune/grid/uggrid/uggridintersections.cc

@@ -633,7 +633,7 @@ int Dune::UGGridLeafIntersection<GridImp>::getFatherSide(const Face& currentFace
   }
 
   // Should never happen
-  return -1;
+  DUNE_THROW(GridError, "Reached code path that should never be reached");
 }
 
 template< class GridImp>