create strings for onion service error pages

For sponsor 27 O2A4 (Better client-side errors), we need strings that can be localized for each onion service error. There are 7 errors listed in ticket:30090#comment:9.

We are implementing error pages that look like Antonela's design at: https://onion-error.glitch.me/

For each of the above errors (XF0-XF6) we need the following strings:

Page/tab title ("Problem loading page" in above design; you can see this when you hover over the tab).
Header (a title; "404 - Onion Not Found" in above design)
Short description (about one line)
Long description (optional; Firefox uses simple HTML for some long descriptions but this may make localization difficult).
Learn more URL (optional)

Note: for XF4 and XF5, users will only see an error page with the above strings if they dismiss the prompt without providing client authorization (see legacy/trac#30237 (moved)).

Tor specs (includes a description of each error):

Related browser and UX tickets: legacy/trac#19251 (moved) legacy/trac#30090 (closed) legacy/trac#23545 (moved)

Pili and Antonela -- can you coordinate with the right people to produce the English strings so we can begin the localization process soon?

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