Better message when using a key sequence as prefix character which is already bound

[Christoph Conrad]

In GNU Emacs 22.0.50.1 (i686-pc-linux-gnu) of 2006-04-27 on brabbelbox

Loading a library which tries to define

(define-key gnus-summary-mode-map "\C-crm" 'gnus-bmk-set)

Emacs message:

"byte-code: Key sequence C-c r m uses invalid prefix characters"

This is because "C-c r" is already bound by me. I think emacs does not
produce an understandable message, especially not understandable for end
users. It should emit a message like

"define-key: key sequence C-c r already bound to `dmacro-wrap-region',
can not be used as prefix character sequence for `gnus-bmk-set'"