[sdb] When a frame is missing it's context, report absent information instead of invalid frame.

	* debugger-agent.c (frame_commands): When a thread is suspended in native code its top frame
	will be missing a context, so it's not possible to inspect it. Instead of reporting an invalid
	frame, it makes more sense to report unavailable data as this is the right clue for clients.
[sdb] Add a test case for returning the right error when a m2n frame has no information.