Show helpful message when purging in-use Field Types

When deleting a FieldType that is in use, the user sees this ugly error and stack trace:
org.springframework.dao.DataIntegrityViolationException: Could not execute JDBC batch update; SQL [delete from field_type where field_type_id=?]; constraint [null]; nested exception is org.hibernate.exception.ConstraintViolationExcept

Stack trace:
org.openmrs.web.controller.form.FieldTypeListController.onSubmit(FieldTypeListController.java:73)
at [ignored] ...
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
<snip>

Instead, the user should see the pink error box with a message saying it can't be deleted. See TRUNK-656 for an example of what to do.