wgengine/wgcfg: always close io.Pipe

In DeviceConfig, we did not close r after calling FromUAPI.
If FromUAPI returned early due to an error, then it might
not have read all the data that IpcGetOperation wanted to write.
As a result, IpcGetOperation could hang, as in #3220.

We were also closing the wrong end of the pipe after IpcSetOperation
in ReconfigDevice.

To ensure that we get all available information to diagnose
such a situation, include all errors anytime something goes wrong.

This should fix the immediate crashing problem in #3220.
We'll then need to figure out why IpcGetOperation was failing.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>
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