err msg, explain the image can simply be missing upstream too

nginx.conf

@@ -170,7 +170,7 @@ http {
         # Block API v1. We dont know how to handle these.
         # Docker-client should start with v2 and fallback to v1 if something fails, for example, if authentication failed to a protected v2 resource.
         location /v1 {
-            return 405 "API v1 is invalid -- you probably need auth to get a v2 endpoint working against $host -- Check the docs";
+            return 405 "API v1 is invalid. Either the image does not exist upstream, or you need auth to get a v2 endpoint working against $host";
         }
 
         # don't cache mutable entity /v2/<name>/manifests/<reference> (unless the reference is a digest)