cli: Name the real Bitcoin Core requirement when it is missing

[BenWestgate]

When bitcoin-cli is not installed, every ms32 command that connects to Bitcoin Core fails with:

Install a reviewed bitcoin-cli before creating a backup.

That includes ms32 secret, ms32 share, ms32 correct and ms32 wallet, and none of them creates a backup. A tester without Core read the message as "I can't create" and didn't understand the codex32 <command> hint printed after it.

• Say what the command uses Core for. secret, share and correct use it to show the master fingerprint and rank corrections. create and wallet give Core the key.
• Say that Bitcoin Core 32 or newer must be running with RPC enabled, and that an unsynced regtest or signet node is enough for practice.
• Keep the codex32 <command> hint and say what that command leaves out, which is the master fingerprint. codex32 has no create, so ms32 create gets no hint.
• ms32 correct connects only after its search finishes, which can take up to ten seconds, and then discards the result. Connect before searching.

Reproduced with no bitcoin-cli on PATH, both on reviewability-v1 @ e5b957a and on a disposable tip that stacks #42, #57, #46, #80 and #81. ms32 secret, ms32 correct (with damaged input), ms32 wallet and ms32 create 2 all print the message above. It comes from BitcoinCore.connect() in _bitcoin_core.py, and the fallback hint comes from _connected_core() in cli.py.

This touches the Core connection path that #57 and #81 also change, so it fits after #81.