* api/render: initial implementation of the package
* acme/api: refactored to support api/render
* authority/admin: refactored to support api/render
* ca: refactored to support api/render
* api: refactored to support api/render
* api/render: implemented Error
* api: refactored to support api/render.Error
* acme/api: refactored to support api/render.Error
* authority/admin: refactored to support api/render.Error
* ca: refactored to support api/render.Error
* ca: fixed broken tests
* api/render, api/log: moved error logging to this package
* acme: refactored Error so that it implements render.RenderableError
* authority/admin: refactored Error so that it implements render.RenderableError
* api/render: implemented RenderableError
* api/render: added test coverage for Error
* api/render: implemented statusCodeFromError
* api: refactored RootsPEM to work with render.Error
* acme, authority/admin: fixed pointer receiver name for consistency
* api/render, errs: moved StatusCoder & StackTracer to the render package
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