Bug Tag editor backend doesn't correct tag case

[Renegade]

On bug pages, when you try to add a tag with uppercase letters, the backend returns an error 400 and does not add any of the tags - the entire request is thrown out.

I, personally, know from my adventures with the importer how the regexp for bug tags looks like, and I, personally, know how to look at the request response to figure out what's happening.

A normal user would be utterly lost.

There is absolutely no indication what is happening and why to the end user: The help text next to the editor does not contain formatting instructions, there's no error message on the page, and since it's a background AJAX request, an end user doesn't see the request and its response either.

In the eyes of an end user, tag input just somehow, inexplicably, doesn't work.

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Expected behavior: Either automatic lowercasing of the submitted string in the backend, or an error message explaining the rejection in the front end, preferably paired with proper formatting instructions for the end user in the help text.

Actual behavior: Quiet failure/Error 400 Bad Request.