Check for that grunt command exists before spawning

script/grunt

@@ -1,9 +1,17 @@
 #!/usr/bin/env node
 var cp = require('./utils/child-process-wrapper.js');
+var fs = require('fs');
 var path = require('path');
 
 // node build/node_modules/.bin/grunt "$@"
 var gruntPath = path.resolve(__dirname, '..', 'build', 'node_modules', '.bin', 'grunt') + (process.platform === 'win32' ? '.cmd' : '');
+
+if (!fs.existsSync(gruntPath)) {
+  console.error('Grunt command does not exist at: ' + gruntPath);
+  console.error('Run script/bootstrap to install Grunt');
+  process.exit(1);
+}
+
 var args = ['--gruntfile', path.resolve('build', 'Gruntfile.coffee')];
 args = args.concat(process.argv.slice(2));
 cp.safeSpawn(gruntPath, args, process.exit);