Custom admincp field error

[Bilderback]

I am trying to create a custom error message to replace the one shown in red border in image below.

http://images.dudditz.com/ext/vb.org/admincp-path-error.gif

The code I am using checks to see if the directory entered is valid.
I would like to show a custom error message to replace the default vbulletin error.
Does anyone know how to do this?
Thanks

[Dismounted]

It is actually a phrase used by all errors that do not pass the AJAX validation. Your best bet would be to explain what can go wrong in the description.