core: Fix panic when trying to decrypt the value for a note with an incorrect viewkey

[moCello]

Describe the bug
When attempting to decrypt the value for an obfuscated note with a view-key that doesn't own the note, the code doesn't error but panics due to entering unreachable code.

To Reproduce
In tests/note_test add:

#[test]
fn note_not_owned() {
    let mut rng = StdRng::seed_from_u64(0xc0b);

    let owner_pk = PublicKey::from(&SecretKey::random(&mut rng));
    let value_blinder = JubJubScalar::random(&mut rng);
    let sender_blinder = [
        JubJubScalar::random(&mut rng),
        JubJubScalar::random(&mut rng),
    ];

    let note = Note::obfuscated(
        &mut rng,
        &owner_pk,
        &owner_pk,
        42,
        value_blinder,
        sender_blinder,
    );

    let not_owner_sk = SecretKey::random(&mut rng);
    let not_owner_vk = ViewKey::from(&not_owner_sk);

    assert!(note.value(Some(&not_owner_vk)).is_err());
}

and run the test:

cargo test --tests note_not_owned

Expected behaviour
In case of an error we should gracefully return the error and not panic.