Clarify err:XD0011 in p:load

[ndw]

The semantics of p:document defer to p:load. In p:load, we find:

It is a dynamic error (err:XD0011) if the resource referenced by a p:load element does not exist or cannot be accessed.

I think we should clarify that if the protocol used to load the document returns an error (404, for example, in the case of http or https), the error should be raised. It's possible to interpret the current wording as meaning that, but we should be more explicit.

At the moment, in my implementation, if you attempt to GET a document that doesn't exist, and the server returns a well-formed error document along with the 404 response, I (incorrectly, I believe) return the error document as the resource.