Handling the attestation pallet's OutStandingAttestationRequest error

[ameba23]

When trying out attestations on a test network, ive had issues with the OutStandingAttestationRequest error. The code all works, but there is maybe a usability issue with our design.

We allow at most one pending attestation request. This means once we have requested an attestation, if we request another one we get an OutStandingAttestationRequest error.

Pending attestations get removed when an attestation is successfully verified, but not if they fail. Which kinda makes sense.

But if our attestation fails with an error, and we change something to fix the issue, we cannot request a new attestation, we have to use the nonce of the existing one.

With the way things are currently set up, all our client functions and test-cli commands assume the happy path where we want to request a fresh attestation. So if something goes wrong we can get in to a sort of locked state where we have no way to remove a pending attestation.

So we need to either:

• Make client functions which take a nonce, or perhaps better still, use the existing pending nonce if there is one, and request a new one otherwise.
• If an attestation fails, remove the pending attestation. That is, you get one chance to submit an attestation, regardless of whether it succeeds or fails.