Added better error checking for file transfer in sip_session

Ignore-this: 5a1f8b3333ad5b7001af6ec47c824f0c

darcs-hash:20091029143721-1e3d8-ac0ad50ae0751da0b4d01399c20e744b5a613f24
Fixed handling of canceled incoming file transfers in sip_session

Ignore-this: 3671dbbbd0fe7dacbf66574fb77c9e6e

darcs-hash:20091030112731-1e3d8-bb22862167a2c50b99f6dc3c3923a43bc5010a18