fix compiler warning
fix compiler warnings
fix compiler warning
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
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).