On incoming call ignore input other than y/n in sip_audio_session.py

darcs-hash:20090417092847-58103-41683c73ce6e4cb0d5158247c6737fee89d8e3d2
made green.core.Error derive from Exception, not from RuntimeError

darcs-hash:20090417104153-b2873-ca173b8fc2e3f89925197609733944aa0331a932