dgram: make send cb act as "error" event handler.

This allows users to provide a callback that handles potential
errors resulting from a `socket.send` call. The original behavior
of emitting the error event on the socket is preserved.

Fixes nodejs#4846.

lib/dgram.js

@@ -304,7 +304,16 @@ Socket.prototype.send = function(buffer,
 
   self._handle.lookup(address, function(ex, ip) {
     if (ex) {
-      if (callback) callback(ex);
+      if (callback) {
+        callback(ex);
+
+        if (self.listeners('error').length) {
+          self.emit('error', ex);
+        }
+
+        return;
+      }
+
       self.emit('error', ex);
     }
     else if (self._handle) {

test/simple/test-dgram-send-cb-quelches-error.js

@@ -0,0 +1,43 @@
+var common = require('../common');
+var assert = require('assert');
+var dgram = require('dgram');
+
+var socket = dgram.createSocket('udp4');
+var buffer = new Buffer('gary busey');
+var order = 0;
+
+var callbackOrder;
+var eventOrder;
+
+// assert that:
+// * callbacks act as "error" listeners if given.
+// * callbacks and error events are emitted in a certain order.
+// 
+socket.send(buffer, 0, buffer.length, 100, 'dne.example.com', callbackOnly);
+
+function callbackOnly(err) {
+  assert.ok(err);
+
+  socket.once('error', onEvent);
+
+  socket.send(buffer, 0, buffer.length, 100, 'dne.example.com', onCallback);
+}
+
+function onEvent(err) {
+  assert.ok(err);
+  eventOrder = order++;
+  check();
+}
+
+function onCallback(err) {
+  assert.ok(err);
+  callbackOrder = order++;
+  check();
+}
+
+function check() {
+  if (eventOrder !== undefined && callbackOrder !== undefined) {
+    assert.ok(eventOrder < callbackOrder);
+    socket.close();
+  }
+}