Warn user that DF logs cannot be viewed, cleared, downloaded while vehicle is armed.

Currently running MP v1.3.56.4 with ArduPlane V3.9.0-dev / ChibiOS firmware on Matek F405-Wing flight controller. FC has integrated SD card reader. DF logging generally works well, as does terrain following. However when user tries to view/clear/download DF logs while armed, MP attempts to do so, and comes back ~30 seconds later with an unfriendly traceback depicted below. Far better, IMHO, to warn user that DF logs cannot be accessed while vehicle is armed.


Reproduce the DF log view, clear, or download actions while the vehicle is armed and inspect the resulting traceback shown in the issue. Trace the Mission Planner entry point for these actions and make the completed behavior an immediate, user-friendly warning instead of the delayed traceback.

Written by the indexing model from the issue text.