server.ts: remove port fallback

If a user has messed up their config so much that the fallbacks in
our default configuration interface don't work then we should just
fail here.

(cherry picked from commit 6c09677)

Author: alanorth

server.ts

@@ -559,7 +559,7 @@ function createHttpsServer(keys) {
  * Create an HTTP server with the configured port and host.
  */
 function run() {
-  const port = environment.ui.port || 4000;
+  const port = environment.ui.port;
   const host = environment.ui.host;
 
   // Start up the Node server