Throw Exception when shape has invalid type

Although the switch statement handles all (current) allowed values, from
a C/C++ view point the enum may have any value of its underlying type.
In this case the function has no return statement and runs into
undefined behavior.

Fixes build failures when built with "-Werror=return-type".

libsrc/occ/python_occ_shapes.cpp

@@ -194,6 +194,7 @@ py::object CastShape(const TopoDS_Shape & s)
     case TopAbs_SHAPE:
       return py::cast(s);
     }
+    throw Exception("Invalid Shape type");
 };