tentative fix for losing (empty) MIDI files. Incomplete because testing shows issues with some workflows
FileSource should not generate its own error message when a file is missing

Conflicts:
	libs/ardour/file_source.cc
fix up condition where a MIDI filesource is marked empty, not found, and its _path member is not set correctly
if a (file) source really cannot be found, differentiate between audio & MIDI.

For now, recovering from missing MIDI is a no-can-do situation (because MIDI is inherently data-editable).
fix API errors after cherry-picking empty MIDI file fixes from cairocanvas