make GreenEngine handle SCEngineGotException and SCEngineDidFail

- rename GreenEngine.link_exception to link_error

darcs-hash:20090320102446-b2873-47739339bca836504189a820e6ff7b678a274e9d
add docstring for play_wav_file

darcs-hash:20090320102556-b2873-6822ca7df6c0cdfd335b82dc897b7c142549e6d5