This is an attempt to fix the problem Max described in his devel mail.
The presence of the rom files will now be checked in detectDevice().
In case of failure there will be fallback attempts.
The user will get notified of the detection failure if he has expressly selected the device that failed.
Please test with your platform / engine (with or without rom files).
The actual MidiDriver derives from it. MidiDriver_BASE only
provides the minimal API necessary for transmitting MIDI data.
The idea is that this is all MidiParser needs, thus allowing
us to simplify the various MidiPlayer classes in our engines.
This in turn enables modifying MidiDriver_MPU401::close() to allow
it to be called on a midi driver that has not yet been opened.
The specific issue that triggered me to make these changes was a
crash-upon-quit in HUGO, caused by it instantiating a midi driver,
then encountering an error (missing hugo.dat) *before* having
opened the new midi driver; the general cleanup code then tries
to close the (not yet opened) midi driver -> kaboom
Also fixed some engines which were leaking MidiDriver instances.
