python warnings breaks worker protocol
python warnings breaks worker protocol

`warnings.showwarning` writes messages to `sys.stderr` breaking disco
worker protocol.

This change wraps any write to stderr the same way stdout is intercepted
by with a MessageWriter instance

mwilliams3/disco,simudream/disco,mwilliams3/disco,simudream/disco,mozilla/disco,pooya/disco,discoproject/disco,seabirdzh/disco,mozilla/disco,pooya/disco,mwilliams3/disco,ErikDubbelboer/disco,seabirdzh/disco,mozilla/disco,seabirdzh/disco,pombredanne/disco,ErikDubbelboer/disco,ktkt2009/disco,ktkt2009/disco,beni55/disco,simudream/disco,oldmantaiter/disco,beni55/disco,ktkt2009/disco,seabirdzh/disco,oldmantaiter/disco,ErikDubbelboer/disco,discoproject/disco,oldmantaiter/disco,pombredanne/disco,pombredanne/disco,mwilliams3/disco,discoproject/disco,beni55/disco,simudream/disco,ktkt2009/disco,beni55/disco,discoproject/disco,ktkt2009/disco,oldmantaiter/disco,mwilliams3/disco,simudream/disco,seabirdzh/disco,oldmantaiter/disco,pooya/disco,pombredanne/disco,discoproject/disco,beni55/disco,ErikDubbelboer/disco,mozilla/disco,pooya/disco,ErikDubbelboer/disco,pombredanne/disco