Update export.go (fix errcheck issue) (#28971)

server/i18n/en.json

@@ -5114,6 +5114,10 @@
     "id": "app.export.export_custom_emoji.copy_emoji_images.error",
     "translation": "Unable to copy custom emoji images"
   },
+  {
+    "id": "app.export.export_custom_emoji.mkdir.error",
+    "translation": "Unable to create a directory for custom emoji images"
+  },
   {
     "id": "app.export.export_write_line.io_writer.error",
     "translation": "An error occurred writing the export data."