sip_session2: don't exit if a command raises an exception

darcs-hash:20090421102112-b2873-347f1499438f011f0130aca93cfab7fa1452307b
sip_session2: more robust handling of incoming files

darcs-hash:20090421102143-b2873-380ad9278e736c3f82fc2794d82191f2bc1ec126