If we get EPIPE on uwrite(), don't close, just do nowrite().

EPIPE doesn't mean the whole socket is dead, it just means we can't write to
it.  Maybe there's still data waiting to be read, though.

ssnet.py

@@ -163,6 +163,11 @@ class SockWrapper:
         try:
             return _nb_clean(os.write, self.wsock.fileno(), buf)
         except OSError, e:
+            if e.errno == errno.EPIPE:
+                debug1('%r: uwrite: got EPIPE\n' % self)
+                self.nowrite()
+                return 0
+            else:
                 # unexpected error... stream is dead
                 self.seterr('uwrite: %s' % e)
                 return 0