Fixed an _all_docs handler bug

If the client POSTed a set of docIDs, and one of those docs was not in
any channels, but the user had access to all channels ("*"), then
_all_docs mistakenly didn't return an entry for that document.
More correct _all_docs handling of unavailable documents

If a document is unavailable, include an entry for it in the response
with the CouchDB "error" property and the CBL "status" property.