health: no-derp-connection warns too early

Starting up tailscaled (at 01a7726cf7306), I see:

```
magicsock: home is now derp-10 (sea)
health(warnable=no-derp-connection): error: Tailscale could not connect to the 'Seattle' relay server. Your Internet connection might be down, or the server might be temporarily unavailable.
[RATELIMIT] format("health(warnable=%s): error: %s")
magicsock: adding connection to derp-10 for home-keep-alive
magicsock: 1 active derp conns: derp-10=cr0s,wr0s
Switching ipn state Starting -> Running (WantRunning=true, nm=true)
health(warnable=warming-up): ok
```

That health warning is distracting. It lasted milliseconds.

The "warming up" stuff kinda papered over it in places (at least for the GUI) but we probably need to do more to avoid log spam too.

/cc @agottardo

Start at tailscaled startup and trace the magicsock health(warnable=no-derp-connection) warning around the transition to Running and warming-up. Reproduce the startup sequence shown in the issue and verify that a transient DERP connection failure no longer emits distracting log output while sustained failures still report health.

Written by the indexing model from the issue text.