override validates message

You could specify an arbitrary attribute name to
ActiveRecord::Errors#add. Arguably, this is an abuse of the API and
may not work in future releases of ActiveRecord.

Alternatively, use ActiveRecord::Errors#add_to_base to add a message
to the base object instead of a specific attribute. This gives you
control of the full error message.

Finally, you could use a localization plugin to handle translation of
error messages in the view.

Ciao,
Sheldon.

http://www.agilewebdevelopment.com/plugins/custom_error_message

ed

it doesn’t lack documentation - see the README file inccluded in the plugin:

Custom Error Message