Mode functions generated via `define-minor-mode' will signal an 
error when messaging a mode's "pretty" name contains "%".

To reproduce:

1. eval: (define-minor-mode 100%-failure-mode "oops")
2. enable or disable the mode.

The line responsible for the error is:

(message ,(format "%s %%sabled%%s" pretty-name) 
         (if ,getter "en" "dis") local)))))

The attached patch fixes the issue for me.