tailscale netcheck makes it sound like DERP won't work

[danderson]

On a UDP-blocked network, tailscale netcheck says this:

 * Nearest DERP: 0 ()
 * DERP latency:

This makes it look like we're saying "DERP doesn't work". In reality what we're saying is "DERP latency checking failed, magicsock will use a random DERP server".

We should be a little more explicit about the meaning of these fields when latency checking fails.

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