Code review comment for lp:~robertcarr/mir/client-focus-notifications

Again with:
437 + case mir_surface_attrib_focus:
438 + notify_change(attrib, value);
439 + break;

It would be easy to write reasonable test cases that now fail. Similar to:
tests/unit-tests/shell/test_surface.cpp: TEST_F(ShellSurface, states)

Because calling:
    surf.configure(mir_surface_attrib_focus, X) != X
and fails to throw an exception too.