There is a high chance of them getting called within the stream servicing function which cannot deal with abnormal conditions, so the only choice here is performing a hard abort.
This was getting a bit unwieldy. The include path setup is not perfect yet, that's work for later.
(It's about time we're getting C++20 with modules so that this include path madness can be put to an end.)
The organization here is now the same as for the Timidity++ device, i.e. it is the device owning the instruments to give better control over their lifecycle.
# Conflicts:
# src/sound/timidity/instrum_font.cpp
# src/sound/timidity/instrum_sf2.cpp
# src/sound/timidity/mix.cpp
# src/sound/timidity/playmidi.cpp
# src/sound/timidity/resample.cpp
