[api spec] Update authN and authZ errors

Associate HTTP 401s with Authentication errors rather than Authorization
errors. Changes the meaning of the UNAUTHORIZED error to be authentication
specific.

Defines DENIED error code to be associated with authorization
errors which result in HTTP 403 responses.

Add 'No Such Repository' errors to more endpoints.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)

cmd/registry-api-descriptor-template/main.go

@@ -53,6 +53,7 @@ func main() {
 		ErrorDescriptors: append(errcode.GetErrorCodeGroup("registry.api.v2"),
 			// The following are part of the specification but provided by errcode default.
 			errcode.ErrorCodeUnauthorized.Descriptor(),
+			errcode.ErrorCodeDenied.Descriptor(),
 			errcode.ErrorCodeUnsupported.Descriptor()),
 	}