Objective 2, Activity 4: Better client-side errors

This is the parent ticket to hold any tickets under this activity, including:

Improving Tor Browser behavior when an onion site supports HTTPS but the HTTPS is not from an approved certificate.
Fixing inconsistent messages we are showing to users accessing .onion sites with self-signed certificates-
Improving Tor Browser’s user experience and error messages when a .onion link fails.
Providing more informative error messages back to the user to better indicate whether the issue was on the service-side, on the client-side, or on the network-side.

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