GUI: Better error messages for minify task in gulp

Change-Id: I25f287ae057ea8f164c5ec7a3f036f3e92dd8f3c
diff --git a/tools/gui/gulp-tasks/bundles/bundle-js/index.js b/tools/gui/gulp-tasks/bundles/bundle-js/index.js
index 90a0e24..212034a 100644
--- a/tools/gui/gulp-tasks/bundles/bundle-js/index.js
+++ b/tools/gui/gulp-tasks/bundles/bundle-js/index.js
@@ -38,6 +38,7 @@
         .pipe(sourceMaps.init())
         .pipe(strip())
         .pipe(uglyfy())
+        .on('error', (e, file, line) => console.error(e))
         .pipe(concat(exportName))
         .pipe(sourceMaps.write('source-map'))
         .pipe(gulp.dest(GUI_BASE + '/dist/'));