Have both sip_audio_session.py and sip_im_session.py disconnect when SDP negotation has failed

darcs-hash:20081128103447-58103-8476f4babee1be0809ef6176d4693e6ea49f8f6a
msrplib bug fixed in MSRPConnector

darcs-hash:20081128081409-b2873-1891f9425a1d5134e17c081b875927453fd21a72