SendNotification returns nil error value in error case

[EmpireJones]

There is at least one case in the SendNotification function of webpush.go in which a nil error value may be returned in an error state:

    // Shared secret
    publicKeyX, publicKeyY := elliptic.Unmarshal(curve, clientPublicKey)
    if publicKeyY == nil {
        return &http.Response{}, err
    }

A new error should be returned for this case; otherwise, the consuming project may incorrectly assume that the Response is valid.

Additionally, the comments from elliptic.Unmarshal indicate that the x value would be null in the case of an error, but this code is checking the y value. This may or may not be ok to do.

[SherClockHolmes]

Thanks again @EmpireJones, I have pushed a fix to master.