Rejected new tag doesn't produce any error message

Problem

There is no error message when trying to create an erroneous tag, from the "New Tag" page.

At the following path: /:user:/:project:/tags/new.

How to reproduce the bug

1. try to create a new tag with a space character in it (let's say "my new tag")
2. click on the "create tag" button

The user is brought back to the tag list. The new tag doesn't appear in the list. There is no error message available.