Clarify bootstrap error when root certificate is not found

[tashian]

New output:

bin/step ca bootstrap --ca-url https://ca:4443 --fingerprint abc123
The request could not be completed: a root certificate with that fingerprint was not found.
Re-run with STEPDEBUG=1 for more info.

[tashian]

I'm not sure if this was actually needed here.

[maraino]

I don't think this is the right place to do it.

We should manage this in the CLI. And in the API side of certificates if we want a better message, currently it returns:

$ curl -s https://localhost:9000/root/foo | jq
{
  "status": 404,
  "message": "The requested resource could not be found. Please see the certificate authority logs for more info."
}