MACOSX: Change assertions to warnings in CoreMIDI driver.

This prevents a device open failure from crashing out ScummVM if the
code which is using this fails to check the return value and tries to
send data to the endpoint.

This is as per the code in the CAMD and several other MIDI drivers.
MACOSX: Change assertions to warnings in CoreAudio driver.

This prevents a device open failure from crashing out ScummVM if the
code which is using this fails to check the return value and tries
to send data to the endpoint.

This is as per the code in the CAMD and several other MIDI drivers.