Improve error message text of "C-x C-SPC"

* lisp/simple.el (pop-global-mark): Mention the buffer name in the
error message.  Suggested by T.V Raman <raman@google.com>.

lisp/simple.el

@@ -6426,7 +6426,8 @@ for it.")
 	     (<= position (point-max)))
 	(if widen-automatically
 	    (widen)
-	  (error "Global mark position is outside accessible part of buffer")))
+	  (error "Global mark position is outside accessible part of buffer %s"
+                 (buffer-name buffer))))
     (goto-char position)
     (switch-to-buffer buffer)))