org-remember: Ignore gnus link if not on a message

This allows you to use org-remember in any gnus buffer - even when
not on a location that will create a usable link.  Instead of
failing with the error "Not on a group" just create a remember
note with no link instead.

This makes org-remember more user-friendly - we don't have to
change buffers to a non-gnus buffer to create a remember note.

lisp/org-gnus.el

@@ -111,12 +111,12 @@ If `org-store-link' was called with a prefix arg the meaning of
 			 (gnus-group-name))
 			(t "???")))
 	   desc link)
-      (unless group (error "Not on a group"))
-      (org-store-link-props :type "gnus" :group group)
-      (setq desc (org-gnus-group-link group)
-	    link desc)
-      (org-add-link-props :link link :description desc)
-      link))
+      (when group
+	(org-store-link-props :type "gnus" :group group)
+	(setq desc (org-gnus-group-link group)
+	      link desc)
+	(org-add-link-props :link link :description desc)
+	link)))
 
    ((memq major-mode '(gnus-summary-mode gnus-article-mode))
     (and (eq major-mode 'gnus-summary-mode) (gnus-summary-show-article))