Explain the reason why certain addons unavailable in certain region

[0x391F]

Some add-ons, such as uBlock Origin https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/, should explain the reason why it unavailable in certain region, for example, China ("law" problem), because some chinese user complain firefox "deny" them to install certain add-ons, and it causing some user turn to use other browsers.

┆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 #1534

[diox]

We do reply with HTTP 451 errors when they add-ons are not available in a region already.