Show a nice error message when Desktop CMU Graphics can't find/handle a sound file

[austin-schick]

Some users (maybe Windows users in particular) see a confusing error like [Errno 22] Invalid argument when trying to open an invalid file.

Long term, the best solution here may be to stop using subprocesses to play sounds, and instead use the PyGame mixer directly. But to close this issue, we jut need to improve this error message.