Jerone

you can achieve this though validation in the form.  Use a new field that is
not displayed on the form.

When the form is submitted its FIRST rule is to the NEWFIELD's value to ON.  
Then setup validation on the form to be conditional on the actual validation
AND that NEWFIELD = ON, that way before the submission to the web service
happens Infopath stops the submission to point out the errors.

Cheers Adam