RFC 8555 § 7.3.5 is not listed as optional but we do not currently
support it. Rather than 404, return a 501 to inform clients that this
functionality is not yet implemented.
The notImplmented error type is not an official error registered in the
ietf:params:acme:error namespace, so prefix if with step:acme:error. An
ACME server is allowed to return other errors and clients should display
the message detail to users.
Fixes: https://github.com/smallstep/certificates/issues/209
