Don't send broken-pipe errors to the Scheme thread when they are

Don't send broken-pipe errors to the Scheme thread when they are
detected by the pipe readahead thread.  Recently, this has seemed to
cause a fatal error under certain timing conditions (which
unfortunately didn't hold while running under the debugger).  Although
this fatal error is now probably fixed, rather than run the risk of
finding out, it seems safer to avoid the behavior that seemed to cause
the problem.  Since the value of reporting broken-pipe errors is
minimal at best, this is not a big deal.