pnet: misleading error errShortNonce during ECONNRESET on Read

[davidhadas]

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.

[sukunrt]

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

That makes sense. Want to open a PR?