Fixed Invitation to not ignore PJMEDIA errors when creating a response

Ignore-this: eb4fbc7cf987b332db28dee9d05e8387

darcs-hash:20100126094505-1e3d8-fa08a8930e4981973c8b7bd39df455e8ee9f5fed
Added handling for PJMEDIA_SDPNEG errors in Session

Ignore-this: ca4152a3626feba56f1612ce71d59ae9

darcs-hash:20100126094532-1e3d8-dac3109e8b32a10e2fc615f0efb11f6a34ec9e36