tools/lookup: fall through if xref backend fails

Part 1 to solution for #4739

modules/tools/lookup/autoload/lookup.el

@@ -188,12 +188,15 @@ This can be passed nil as its second argument to unset handlers for MODES. e.g.
                         (xref-find-backend)
                         identifier)))
     (when xrefs
+      (let ((marker-ring (ring-copy xref--marker-ring)))
         (funcall (or show-fn #'xref--show-defs)
                  (lambda () xrefs)
                  nil)
         (if (cdr xrefs)
             'deferred
-        t))))
+          ;; xref will modify its marker stack when it finds a result to jump to.
+          ;; Use that to determine success.
+          (not (equal xref--marker-ring marker-ring)))))))
 
 (defun +lookup-dictionary-definition-backend-fn (identifier)
   "Look up dictionary definition for IDENTIFIER."