Objective 2, Activity 2: Notify users about typo errors when entering .onion addresses

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

• Using the address format of onion services v3 that allows us to detect typos.
• Experimenting with the optimal user experience for this error case, e.g. offering a retry-button after explaining what went wrong.
• Implementing a special error page that tells the user the problem is a typo in the address.