VC-AuthN controller throws error when agent does not have a DID

[esune]

In the event that the agent instance used by VC-AuthN does not have a DID (e.g: the user forgot to start the agent providing a seed), the controller throws an http 500 error without further details.

Acceptance Criteria:
When an event causes the controller to be unable to complete the request, it should be logged with a meaningful message.

[stale]

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