Define errors for main D-Bus API

[iinuwa]

We need to go through the codebase and remove all unwraps and define actual errors.
We should take care to avoid leaking too much data to the client and introduce privacy concerns.

[iinuwa]

Started work on this in #47

[iinuwa]

It looks like the only errors that is not being passed from Credential service to the gateway is the InvalidStateError.

I don't think that ConstraintError should ever make it back to the caller, as it's only thrown by individual authenticators, not the platform request.

All the other types of errors from the WebAuthn spec (AbortError, SecurityError, TypeError and the catch-all NotAllowedError)) should now be passed properly back to the gateway client.