Language and aref hooks for other specs

After thinking a bit about general editorial issues in the protocol, I think the issue that got most prominent in my mind was that we need more hooks for other specs to reference when they need to. Let me take a concrete example:

In the interop spec, they say e.g.:

1. MUST return 404 if TREE or REGISTRY cannot be successfully dereferenced.

This is actually and additional restriction on the implementations of Solid servers (as the protocol specifies that amongst other things, a 410 may be returned if the resource has previously existed), which shouldn't be done, and shouldn't be necessary to do.

Instead, specs like the interop specs should have terms to point to, something like "return a non-existent resource error if TREE or REGISTRY cannot be successfully dereferenced.", and then, "non-existent resource error" is a term that we should use in the protocol spec to refer to that kind of situation, and then say that it would mean 404 or 410 or whatever. Those terms should also have ids, so that they can link right to them.

This, I hope, will enable other specs to not define anything that has to do with the protocol, and so enable orthogonality.

Start by reviewing the protocol specification's existing error terminology and the interop specification's TREE and REGISTRY dereferencing requirement. Define the cross-specification terms, linkable ids, and status-code semantics needed so related specifications can reference the protocol without redefining it.