Make handling credential issuance more resilient to network failures

[dlongley]

If a credential issuance request is successfully transmitted to an issuer instance and that issuer instance successfully issues a credential -- but an error occurs in transmitting the response, the workflow instance should handle this more gracefully. If a duplicate error is returned, the workflow instance should retrieve the already issued VC. Note: For ephemeral VCs (i.e., without status or IDs), issuance should already succeed on retry.

[dlongley]

Related: digitalbazaar/bedrock-vc-issuer#143