Add a removing reason to the page "Oops! We can’t find that page"

[Juraj-Masiar]

Describe the problem and steps to reproduce it:

Visit addon page:
// removed
https://addons.mozilla.org/en-US/firefox/addon/flash-videodownloader/
// probably author removed
https://addons.mozilla.org/en-US/firefox/addon/enhancer-for-youtube/
// author disabled
https://addons.mozilla.org/en-US/firefox/addon/fast_firefox_send/
// non-existing
https://addons.mozilla.org/en-US/firefox/addon/asdffdalkdfjklsajdflkjasdlkfj/

What happened?

It says either Mozilla removed it or author removed it.

What did you expect to happen?

It should say the truth!
For example: "Removed by Mozilla", "Removed by author", "Invalid link"

┆Issue is synchronized with this Jira Task

[rugk]

Also, you may want to reply with HTTP 451 when it has been removed due to legal reasons. After all “451 Unavailable For Legal Reasons” already explains a lot.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/451

Ref coming from this issue/background, where exactly that happened.
Ref #1561

[diox]

As replied in #1561 we do reply with HTTP 451 for regional legal restrictions.