fix nil pointer panic in rpc backoff logic (#3770)
fix nil pointer panic in rpc backoff logic (#3770)

If we don't get a session within the context we should error out instead of trying to OpenStream on a nil session.
Oncilla/scion,Oncilla/scion,netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion