Gracefully handle MIDI parse failure.
Load what we can from broken/truncated MIDI files.

We're still a very long way from tolerant of weird SMF files (libsmf takes a
"crash if input is not exactly perfect" philosophy, if we're going to be polite
and elevate such a thing to "philosophy"), but at least we'll get what's there
from files truncated by old broken versions of Ardour or other situations.