ipn: warn about self as the exit node if backend is running (#17018)

Before:

    $ tailscale ip -4
    1.2.3.4

    $ tailscale set --exit-node=1.2.3.4
    no node found in netmap with IP 1.2.3.4

After:

    $ tailscale set --exit-node=1.2.3.4
    cannot use 1.2.3.4 as an exit node as it is a local IP address to this machine; did you mean --advertise-exit-node?

The new error message already existed in the code, but would only be
triggered if the backend wasn't running -- which means, in practice,
it would almost never be triggered.

The old error message is technically true, but could be confusing if you
don't know the distinction between "netmap" and "tailnet" -- it could
sound like the exit node isn't part of your tailnet. A node is never in
its own netmap, but it is part of your tailnet.

This error confused me when I was doing some local dev work, and it's
confused customers before (e.g. #7513). Using the more specific error
message should reduce confusion.

Updates #7513
Updates https://github.com/tailscale/corp/issues/23596

Signed-off-by: Alex Chan <alexc@tailscale.com>

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