Requier explicit dir param and suppress stupid "error" messages.
Add support for saving "non-belonging" a/v note waypoints to a file.

My usecase is recovering at least waypoints from a trip where OsmAnd
crashed and thus track was lost.
Don't crash if underlying media file doesn't exist, just remove waypoint.

The usecase here is that user goes thru media content and removed bad
pictures/recordings, hence a waypoint in GPX should be gone too.