How can I change message set on form.inc?
It's notoriously difficult to target/replace specific messages in the Drupal system. It's not impossible but it's a bit 'messy'.

I'd strongly recommend instead that you install the String Overrides module and use that to replace the specific message instead. String Overrides:


  Provides a quick and easy way to replace any text on the site.


It is available for any string in the system which is passed through the t() function, which all core error messages are.