_nb_clean: don't catch EPIPE after all.

EPIPE is a serious error from these places, so we have to actually do
something.  Otherwise the client ends up busy waiting when the server
disconnects by surprise.

Bug noticed in a log from Chetan Kunte.

ssnet.py

@@ -51,7 +51,7 @@ def _nb_clean(func, *args):
     try:
         return func(*args)
     except OSError, e:
-        if e.errno not in (errno.EWOULDBLOCK, errno.EAGAIN, errno.EPIPE):
+        if e.errno not in (errno.EWOULDBLOCK, errno.EAGAIN):
             raise
         else:
             debug3('%s: err was: %s\n' % (func.__name__, e))