Handle peer 'error' events correctly

Handle peer 'error' events that are fired *before* the peer is emitted
in a 'peer' event. Once the peer is emitted in a 'peer' event, then
it's the consumer's responsibility to listen for errors.

This fixes the most common error in WebTorrent Desktop according to our
telemetry.

Author: feross

lib/client/websocket-tracker.js

@@ -276,7 +276,7 @@ WebSocketTracker.prototype._onAnnounceResponse = function (data) {
   var peer
   if (data.offer && data.peer_id) {
     debug('creating peer (from remote offer)')
-    peer = new Peer({
+    peer = self._createPeer({
       trickle: false,
       config: self.client._rtcConfig,
       wrtc: self.client._wrtc
@@ -389,7 +389,7 @@ WebSocketTracker.prototype._generateOffers = function (numwant, cb) {
   function generateOffer () {
     var offerId = randombytes(20).toString('hex')
     debug('creating peer (from _generateOffers)')
-    var peer = self.peers[offerId] = new Peer({
+    var peer = self.peers[offerId] = self._createPeer({
       initiator: true,
       trickle: false,
       config: self.client._rtcConfig,
@@ -419,4 +419,28 @@ WebSocketTracker.prototype._generateOffers = function (numwant, cb) {
   }
 }
 
+WebSocketTracker.prototype._createPeer = function (opts) {
+  var self = this
+  var peer = new Peer(opts)
+
+  peer.once('error', onError)
+  peer.once('connect', onConnect)
+
+  return peer
+
+  // Handle peer 'error' events that are fired *before* the peer is emitted in
+  // a 'peer' event.
+  function onError (err) {
+    self.client.emit('warning', new Error('Connection error: ' + err.message))
+    peer.destroy()
+  }
+
+  // Once the peer is emitted in a 'peer' event, then it's the consumer's
+  // responsibility to listen for errors, so the listeners are removed here.
+  function onConnect () {
+    peer.removeListener('error', onError)
+    peer.removeListener('connect', onConnect)
+  }
+}
+
 function noop () {}