TorBrowser might want to have an error page specific to when .onion links fail

When a webpage fails to load, it can be due to any number of factors. But when that page is served by an onion service, some of those factors have greater implications for security.

It would be cool to know if the failure is related to a malfunction in the local Tor instance, or due to nonlocal failures. If TBB can determine this, adding something to TBB to create .onion specific error messages with greater detail would be helpful.