When validating an ACME challenge (`device-attest-01` in this case,
but it's also true for others), and validation fails, the CA didn't
return a lot of information about why the challenge had failed. By
introducing the ACME `Subproblem` type, an ACME `Error` can include
some additional information about what went wrong when validating
the challenge.
This is a WIP commit. The `Subproblem` isn't created in many code
paths yet, just for the `step` format at the moment. Will probably
follow up with some more improvements to how the ACME error is
handled. Also need to cleanup some debug things (q.Q)
