Exception thrown by validate() method of custom Validator handled as a server error

I've made a custom Validator for workflow transitions that checks that a given field isn't left empty.
As required, the validate() method of the Validator throws an InvalidInputException when the user leaves the field empty.

Problem: this exceptions is treated as a server error by JIRA and written in the log files and on the console!

Bad user input is not a server error, it is a normal use case... I'd like my validator to behave just like a normal date/time or username validator.

Thanks for any help...
Rémy.