ffmpeg operations should stop when add-on removed/reloaded

[mi-g]

Currently, if an operation like an aggregation is in progress and the add-on is reloaded, hence not monitoring the operation anymore, the ffmpeg processes keep working in memory.

If the add-on receives a progress update from a coapp operation it does not known, it should return an exception in the progress rpc function so that the coapp knows it's time to kill the ffmpeg process.