pnet: misleading error `errShortNonce` during ECONNRESET on Read

[davidhadas]

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While using a PrivareNetwork (psk) between peers, a general errShortNonce error is returned: failed to negotiate security protocol: privnet: could not read full nonce The actual reason for the failure is obscured.

For example, when the connection was reset (probably by the peer) .

As a minimum the original error (ECONNRESET in this case) should be wrapped as part of the retuned error.