Add label in error message

Author: naokazuterada

markdowntoc/base.py

@@ -24,6 +24,7 @@ def log(self, arg):
             pp.pprint(arg)
 
     def error(self, arg):
+        arg = 'MarkdownTOC Error: '+arg
         arg = str(arg)
         sublime.status_message(arg)
         pp.pprint(arg)