sip_session2: fix sending a message

darcs-hash:20090416132713-b2873-577f46922c9dde9ebc515fb13e2ebe101506773a
sip_session: don't exit if call raises SIPCoreError

darcs-hash:20090417073822-b2873-e0ec794706003dbed26c49805d5de6c43f155872