The 503 synth and 500 minimal response status codes are too misleading
in this context, where the failure is attributed to the client. Among
existing 4XX status codes, this is the closest if we stretch the timeout
definition to "didn't complete rapidly enough before the client went
away".

Conflicts:
	bin/varnishd/cache/cache_req_fsm.c

There is no minimal 500 response on this branch.