[FIX] handle websocket errors in browsers

emit not RippledNotInitializedError if server doesn't have any
completed ledgers on connect

test/utils/net-utils.js

@@ -0,0 +1,26 @@
+'use strict'; // eslint-disable-line 
+
+const net = require('net');
+
+// using a free port instead of a constant port enables parallelization
+function getFreePort() {
+  return new Promise((resolve, reject) => {
+    const server = net.createServer();
+    let port;
+    server.on('listening', function() {
+      port = server.address().port;
+      server.close();
+    });
+    server.on('close', function() {
+      resolve(port);
+    });
+    server.on('error', function(error) {
+      reject(error);
+    });
+    server.listen(0);
+  });
+}
+
+module.exports = {
+  getFreePort
+};