bitcoin-cli: Make error message less confusing
bitcoin-cli: Make error message less confusing

Sorry for the churn on this, but the current message (introduced in #9073)
isn't acceptable:

    $ src/bitcoin-cli getinfo
    rpc: couldn't connect to server
    (make sure server is running and you are connecting to the correct RPC port: -1 unknown)

Putting the error code after the words "RPC port" made me wonder whether
there was a port configuration issue.

This changes it to:

    $ src/bitcoin-cli getinfo
    error: couldn't connect to server: unknown (code -1)
    (make sure server is running and you are connecting to the correct RPC port)

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