webserver: Emit websocket outputError and show error on the landing page.

Whenever outputError is emitted by Ember, display error message on the landing page
and stop the page from refreshing every four seconds.

Refs: https://github.com/embark-framework/embark/issues/946

lib/modules/webserver/server.js

@@ -50,6 +50,11 @@ class Server {
           ws.send('outputDone');
         });
       });
+      self.events.on('outputError', () => {
+        if (ws.readyState === WEB_SOCKET_STATE_OPEN) {
+          return ws.send('outputError');
+        }
+      });
     });
 
     async.waterfall([

lib/modules/webserver/templates/embark-building-placeholder.html.ejs

@@ -30,6 +30,9 @@
       ws.addEventListener('message', (evt) => {
         if(evt.data === 'outputDone') {
           location.reload(true);
+        } else if(evt.data === 'outputError') {
+          document.getElementsByClassName("loading-msg")[0].innerHTML = "Error building dapp, please check console";
+          window.stop()
         }
       });
     </script>