Normalize a response for a missing document's doc info

On a missing document couch_db:open_doc/2 returns a tuple
`{not_found, missing}`, but couch_db:get_doc_info/2 and
couch_db:get_full_doc_info/2 return just an atom not_found.

This change normalizes the response from all three functions
in fabric to be the tuple `{not_found, missing}`