Update build.js

I suggest more explicit error message on gulp build command fail, so developer will have an initial idea about what's wrong. Here's a related discussion and bug report: https://github.com/Semantic-Org/Semantic-UI/issues/2056

tasks/build.js

@@ -16,7 +16,7 @@ module.exports = function(callback) {
   console.info('Building Semantic');
 
   if( !install.isSetup() ) {
-    console.error('Cannot build files. Run "gulp install" to set-up Semantic');
+    console.error('Cannot find semantic.json file. Run "gulp install" to set-up Semantic');
     return;
   }
 
@@ -31,4 +31,4 @@ module.exports = function(callback) {
   gulp.start('build-assets');
 
 
-};
+};