Generate user friendly message for the case when port is occupied. Do…

… not generate unhandled exception if noone has attached `error` handler to the `LegacyPlatformProxifier`.

src/core/cli/commands/serve.ts

@@ -21,6 +21,7 @@ import chalk from 'chalk';
 import { isMaster } from 'cluster';
 import { accessSync, constants as fsConstants } from 'fs';
 import { resolve } from 'path';
+import typeDetect from 'type-detect';
 import { Arguments, CommandModule, Options } from 'yargs';
 import { getConfig } from '../../../server/path';
 import { fromRoot } from '../../../utils/from_root';
@@ -244,7 +245,7 @@ export function createServeCommand(): CommandModule {
         }
 
         if (argv.port !== undefined && isNaN(argv.port)) {
-          throw new Error(`[port] must be a number, but was a string`);
+          throw new Error(`[port] must be a number, but got ${typeDetect(argv.port)}`);
         }
 
         return true;
@@ -276,7 +277,7 @@ function handleServeCommand(cliArgs: Arguments, installationFeatures: Installati
       onShutdown: (reason?: Error | string) => {
         if (reason !== undefined) {
           // tslint:disable no-console
-          console.error(`\n${chalk.white.bgRed(' ERROR ')} ${reason}\n`);
+          console.error(`\n${chalk.white.bgRed(' ERROR ')} ${formatErrorMessage(reason)}\n`);
         }
 
         process.exit(reason === undefined ? 0 : (reason as any).processExitCode || 1);
@@ -302,3 +303,11 @@ function handleServeCommand(cliArgs: Arguments, installationFeatures: Installati
   process.on('SIGINT', () => shutdown());
   process.on('SIGTERM', () => shutdown());
 }
+
+function formatErrorMessage(reason: any) {
+  if (reason.code === 'EADDRINUSE' && Number.isInteger(reason.port)) {
+    return `Port ${reason.port} is already in use. Another instance of Kibana may be running!`;
+  }
+
+  return reason;
+}

src/core/server/legacy_compat/legacy_platform_proxifier.ts

@@ -51,6 +51,11 @@ export class LegacyPlatformProxifier extends EventEmitter {
         return [
           eventName,
           (...args: any[]) => {
+            // We don't want to generate unhandled exception if there are no `error` listeners.
+            if (eventName === 'error' && this.listeners(eventName).length === 0) {
+              return;
+            }
+
             this.log.debug(`Event is being forwarded: ${eventName}`);
             this.emit(eventName, ...args);
           },