This should implement error page navigation similar to native, which is unfortunately mostly undocumented.
Briefly, native uses a local resource in some DLL (the exact one varies by Windows version and locale, so it's not reliable to test) which has a bunch of html error pages for various errors (404 is a different resource than 405, etc). In Windows XP this is in `shdoclc.dll` (which is where I placed it as well, because we already use it in Wine), in Windows 10 it's somewhere in `system32\en-US\ieframe.dll` (or something like that), for US locale at least.
To keep it simple for now, this only adds one error page called `ERROR.HTM` though, which is generic and displays the error code (given via a query). That said, it should still probably be localized somehow, but I've no idea how to localize html resources…
The error page is navigated with URL of the form `res://C:\windows\system32.../NAME.HTM#original_url`, and some of the notifications are special cased to return the original_url from the fragment (on native as well, tests included).
-- v2: ieframe: Implement DocObjectService_IsErrorUrl. ieframe: Send the original URL for error pages in FireDocumentComplete ieframe: Send the original URL for error pages in FireNavigationComplete2 ieframe: Use the original URL for error pages' address bar. ieframe: Navigate to a basic error page on failed navigation. ieframe: Implement IEHTMLWindow2_get_document. shdoclc: Add a basic generic html page for navigation errors. mshtml: Implement SuperNavigate undocumented flag 2.