next (final?) part of handling missing MIDI files.

If an external-to-session file is missing, consider it a fatal error in session loading.
If an internal-to-session file is missing, just create a new MIDI source with the same path and ID, and use that instead.
next (final?) part of handling missing MIDI files.

If an external-to-session file is missing, consider it a fatal error in session
loading.

If an internal-to-session file is missing, just create a new MIDI source with
the same path and ID, and use that instead.