fix(PeerGroup): handle lack of WebRTC support

When WebRTC is not supported (old browsers, or in Node.js without the "wrtc" package installed, don't try to get WebTC peers from a web seed. Also, throw an error if we try to listen for web connections.

lib/peerGroup.js

@@ -13,6 +13,17 @@ var u = require('./utils.js')
 
 var webSeeds = require('./constants.js').webSeeds
 
+var supportsWebRTC = false
+if (process.browser) {
+  var RTCPC = window.mozRTCPeerConnection || window.webkitRTCPeerConnection || window.RTCPeerConnection
+  supportsWebRTC = !!RTCPC
+} else {
+  try {
+    require('wrtc')
+    supportsWebRTC = true
+  } catch(e) {}
+}
+
 // HACK: suppress warnings from Buffer#get()
 Buffer.prototype.get = function get (offset) {
   return this.readUInt8(offset)
@@ -151,14 +162,16 @@ PeerGroup.prototype.connect = function (opts, cb) {
     })
   }
 
-  webSeeds.forEach(function (uri) {
-    var client = new PeerhubClient(uri, function () {
-      self.webSeeds.push(client)
-      self.emit('seedconnect', client)
-      self._connectToWebPeers()
-      if (self.acceptWeb) self._acceptFromPeerhub(client)
+  if (supportsWebRTC) {
+    webSeeds.forEach(function (uri) {
+      var client = new PeerhubClient(uri, function () {
+        self.webSeeds.push(client)
+        self.emit('seedconnect', client)
+        self._connectToWebPeers()
+        if (self.acceptWeb) self._acceptFromPeerhub(client)
+      })
     })
-  })
+  }
 }
 
 PeerGroup.prototype._connectToLocalhost = function (cb) {
@@ -231,6 +244,7 @@ PeerGroup.prototype._onWebPeerConnect = function (conn, incoming) {
 }
 
 PeerGroup.prototype.acceptWebPeers = function () {
+  if (!supportsWebRTC) throw new Error('WebRTC is not supported')
   this.webSeeds.forEach(this._acceptFromPeerhub.bind(this))
   this.acceptWeb = true
 }