Provide custom template error message to ACME clients

[F13]

Custom template errors from fail don't get passed back to ACME clients. For example:

{{- if typeIs "*rsa.PublicKey" .Insecure.CR.PublicKey }}
        {{ fail "No RSA keys please :)" }}

yields

{"type":"urn:ietf:params:acme:error:serverInternal","detail":"The server experienced an internal error"}

using acme.sh.

This would be great to inform clients why their certificate was rejected if there was some custom logic to do so.

[dopey]

Hey @F13 thanks for opening the issue. Error propagation is definitely a deficiency.

@maraino you mentioned you could point me to where templating errors are returned. If this is an easy / quick fix I could try to knock it out.

[maraino]

@dopey It's here, as the error is wrapped you will need to look at the base error https://github.com/smallstep/certificates/blob/8d229b9a60a8e58c93e2479a7c9547ebeaf7d16f/authority/tls.go#L116-L121