[Bug]: Handle when Blocktank http server is unreachable

[catch-21]

Describe the bug

If Blocktank http server is down or in a state of error and cannot be contacted (nginx returning 502) then Bitkit doesn't warn or prevent any user actions. It allowed me to begin creating a CJIT LN connection, although it presented 0 sats minimum and $0 fees (see attached screenshots).

It would be good if Bitkit handled this better, such as preventing anything relating to Lightning when it cannot confirm healthy Blocktank.

Reproduce

1. With no existing LN connection (http server was down so https://api1.blocktank.to/api/info returned 502 from nginx)
2. With Blocktank down
3. Receive an instant payment
4. Observe 0 minimum at value entry page (screenshot 1)
5. Continue and observe £0 service provider fee (screenshot 2)

Screenshots / Recording

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Operating system

Android 13 TKQ1.221114.001

Bitkit version

658b9f2bd22d2ca63893a89dc56f47a054546d79

Log output

No response