I have a CustomValidator that is validating a telephone number for several different telephone numbering schemes. The client-side javascript looks like this:
It looks like your using jquery so you could set the error message like this:
$(sender).text("* Not a NA Phone #");
Sorry for the delay but I was away on vacation.
I was playing a round with a simple page and I think I understand how this works.
When you set the errormessage on the sender in the client side validation function you are setting the equivalent of the ErrorMessage property. This is different from the Text property. The ErrorMessage sets the text displayed in the Validation summary, while the Text property is the text displayed in the validator control. Note that if you only set the ErrorMessage property (on the server) then the ErrorMessage will be copied into the Text property.
What this means is that when you set sender.errormessage your actually setting the text that is displayed in the validation summary.
Hopefully this will help clear it up. However, I still haven't seen a way to change the Text property from the client side validation function. I am also not sure why your sender object is not the validator object.