Add NotImplementedError.

py/qstrdefs.h

@@ -36,6 +36,7 @@ Q(IndentationError)
 Q(IndexError)
 Q(KeyError)
 Q(NameError)
+Q(NotImplementedError)
 Q(OSError)
 Q(SyntaxError)
 Q(TypeError)

py/runtime.c

@@ -177,6 +177,7 @@ void rt_init(void) {
     mp_map_add_qstr(&map_builtins, MP_QSTR_OverflowError, mp_obj_new_exception(MP_QSTR_OverflowError));
     mp_map_add_qstr(&map_builtins, MP_QSTR_OSError, mp_obj_new_exception(MP_QSTR_OSError));
     mp_map_add_qstr(&map_builtins, MP_QSTR_AssertionError, mp_obj_new_exception(MP_QSTR_AssertionError));
+    mp_map_add_qstr(&map_builtins, MP_QSTR_NotImplementedError, mp_obj_new_exception(MP_QSTR_NotImplementedError));
     mp_map_add_qstr(&map_builtins, MP_QSTR_StopIteration, mp_obj_new_exception(MP_QSTR_StopIteration));
 
     // built-in objects