Improve discovery error screen

[komret]

Discovery errors are a common issue Trezor support interacts with. Let's improve this screen so that users can help themselves.

Examples of reported cases:
https://www.reddit.com/r/TREZOR/comments/1g6yxww/account_discovery_error/
https://www.reddit.com/r/Bitcoin/s/Oe2cHVmvNJ

What we could do better:

• Do not show 0 balance in the account bar so that users don't freak out.
• Add a hint that the error may be caused by the user's VPN. That seems to be the most common cause.
• If Tor is on, reconnect to Tor on clicking "Retry". It is possible that a Tor exit node is blocked by our servers. (Desktop app only)
• If Retry does not help, add a switch to disable/enable Tor. (Desktop app only)

Current state:

[bosomt]

• tried to replicate discovery fails after update problem multiple times with no luck /remembered and not remembered wallet/
• but All backends are down happens from time to time but not so often compared with old Suite version
• some button like reset app or remove wallet /restart discovery button would be great

[tomasklim]

Another hint:

• user is using custom backend

On Opakovat we can try to switch between our backends (from eth1 to eth2 and vice versa)

[tomasklim]

related #12911

[sime]

Do not show 0 balance in the account bar so that users don't freak out.

The accounts bar has three states, the error state is not ideal

Happy

Show account balance

Loading

Account not visible

Error

Show broken/error account
Causes:

• Unable to access blockbook
• Custom backend not available