message: gracefully quit server when watchman service is missing
diff: import yargs from 'yargs';
  import client from './client';
  import {startServer} from 'graphql-language-service-server';
  +import watchman from 'fb-watchman';
  const {argv} = yargs
  .usage(
  @@ -89,6 +90,16 @@ const command = argv._.pop();
  switch (command) {
  case 'server':
  + const watchmanClient = new watchman.Client();
  + watchmanClient.capabilityCheck({}, (error, res) => {
  + if (error) {
  + process.stderr.write(
  + `Cannot find installed watchman service with an error: ${error}`,
  + );
  + process.exit(0);
  + }
  + });
  +
  const options = {};
  if (argv && argv.port) {
  options.port = argv.port;
  