Fixed ti tag, shouldnt dump stacktrace

- `ti tag` was throwing a stacktrace instead of failing gracefully.
  parser() was being called [incorrectly], but usage() is the
  correct command.

lib/ticgit-ng/command/tag.rb

@@ -20,7 +20,7 @@ def execute
         else
           puts 'You need to at least specify one tag to add'
           puts
-          puts parser
+          puts usage
         end
       end
     end