Add build-raw-files log; fixes #12603

Fixes #12672

grunt/bs-raw-files-generator.js

@@ -6,8 +6,9 @@
  * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)
  */
 'use strict';
-var btoa = require('btoa');
 var fs = require('fs');
+var btoa = require('btoa');
+var grunt = require('grunt');
 
 function getFiles(type) {
   var files = {};
@@ -27,5 +28,12 @@ module.exports = function generateRawFilesJs(banner) {
     banner = '';
   }
   var files = banner + getFiles('js') + getFiles('less') + getFiles('fonts');
-  fs.writeFileSync('docs/assets/js/raw-files.min.js', files);
+  var rawFilesJs = 'docs/assets/js/raw-files.min.js';
+  try {
+    fs.writeFileSync(rawFilesJs, files);
+  }
+  catch (err) {
+    grunt.fail.warn(err);
+  }
+  grunt.log.writeln('File ' + rawFilesJs.cyan + ' created.');
 };