end() throws exception if drone has already landed, or lost power, through mishap

[ceperman]

end() attempts to land the drone normally and doesn't take into account that the flight may have ended badly e.g. crashed (i.e. already landed) or worse (crashed and battery popped out - this has happen to me on a number of occasions). In both of these situations, end() will hang retrying land() and eventually throw an exception. This function is typically called during normal application termination, but the exception means that the application may never close. Ditto the enclosed streamoff() command.

end() would be more robust if it did not expect a response from its communication with the drone or used a much reduced timeout, and did not raise an exception if it failed.