Fix #786 delayed tlog upload

* Don't upload if username/psw not specified
* Reword password description in preferences
* If server returns an error on any upload, stop all uploads until next attempt (in droneapi.jar)
* Auto cancel notification on pick
Server now returns error code 406 for boring flights.

In that case, client should just kill the notification and move on.
Merge pull request #787 from geeksville/dapiFix

Fix #786 delayed tlog upload