Scary error message displayed if connection is lost while using MAVFTP

This scary looking error message is displayed if MP loses contact with the autopilot while the MAVFTP window is open. The particular case that I saw this was when [uncovering a potential but in the AP flight code related to MAVFTP](https://github.com/ArduPilot/ardupilot/issues/13536) but I suspect the message appears under any situation in which the connection is lost (i.e. the user accidentally disconnects the USB cable or telemetry radio).

We should display something nice like, "Connection lost" instead of this scary message.

Start at the MAVFTP window and reproduce the issue by disconnecting the autopilot or USB/telemetry connection while it is open. Trace the connection-loss handling and replace the scary error presentation with a clear “Connection lost” message, then verify the window behaves correctly after contact is lost.

Written by the indexing model from the issue text.