streams2: Still emit error if there was a write() cb

lib/_stream_writable.js

@@ -175,8 +175,10 @@ function onwrite(stream, er) {
         });
       else
         cb(er);
-    } else
-      stream.emit('error', er);
+    }
+
+    // backwards compatibility.  still emit if there was a cb.
+    stream.emit('error', er);
     return;
   }
   state.length -= l;