For ACME errors, return the internal error string instead of the
ACME one on the "Error() string" function. This way the logs will
have more information about the cause of an error.
Fixes#1057
* 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
