improved error reporting in sip_im_send_file.py

darcs-hash:20081128160735-b2873-ddce5cf02e0248b5da6fcdbf5578f982f8117cd1
do no attempt to send messages in file transfer session

darcs-hash:20081128160945-b2873-5709b6868c2f6856675b8b3aac5b023a9a351410