Transaction t = s.beginTransaction();
      try {
         s.save(e);
         t.commit();
         System.out.println("OK");
      } catch (Exception ex) {
         System.out.println("ERROR");
         t.rollback();
      }

ERROR
OK

CREATE TABLE "public"."event" (
  "eventid" INTEGER NOT NULL,
  "textlabel" CHAR(25) NOT NULL,
  "groupid" INTEGER NOT NULL,
  "timewhen" TIMESTAMP WITHOUT TIME ZONE NOT NULL,
  "timeuntil" TIMESTAMP WITHOUT TIME ZONE NOT NULL,
  "eventroomid" INTEGER,
  "membersonly" BOOLEAN DEFAULT false NOT NULL,
  CONSTRAINT "event_pkey" PRIMARY KEY("eventid"),
  CONSTRAINT "event_fk" FOREIGN KEY ("eventroomid")
    REFERENCES "public"."eventroom"("eventroomid")
    ON DELETE NO ACTION
    ON UPDATE NO ACTION
    NOT DEFERRABLE,
  CONSTRAINT "event_fk1" FOREIGN KEY ("groupid")
    REFERENCES "public"."group"("groupid")
    ON DELETE NO ACTION
    ON UPDATE NO ACTION
    NOT DEFERRABLE
) WITHOUT OIDS;

CREATE RULE "IllegalTimeRangeInsert" AS ON INSERT TO "public"."event"
WHERE (( SELECT count(*) AS count
           FROM event
          WHERE new.eventroomid = event.eventroomid AND new.eventid <> event.eventid AND (new.timewhen >= event.timewhen AND new.timeuntil <= event.timeuntil OR new.timewhen <= event.timewhen AND new.timeuntil >= event.timeuntil OR (new.timeuntil - '00:00:01'::interval) >= event.timewhen AND (new.timeuntil - '00:00:01'::interval) <= event.timeuntil OR new.timewhen >= event.timewhen AND new.timewhen <= (event.timeuntil - '00:00:01'::interval)))) > 0
DO (SELECT exception('EventCollision'::character varying) AS exception;);

CREATE RULE "IllegalTimeRangeUpdate" AS ON UPDATE TO "public"."event"
WHERE (( SELECT count(*) AS count
           FROM event
          WHERE new.eventroomid = event.eventroomid AND new.timewhen > event.timewhen AND new.timeuntil < event.timeuntil OR new.timewhen < event.timewhen AND new.timeuntil > event.timeuntil OR new.timeuntil > event.timeuntil AND new.timeuntil < event.timewhen OR new.timewhen > event.timewhen AND new.timewhen < event.timeuntil)) > 0
DO (SELECT exception('${err.EventCollision}'::character varying) AS exception;);

CREATE RULE "negativeExceptionInsert" AS ON INSERT TO "public"."event"
WHERE new.timewhen > new.timeuntil
DO (SELECT exception('${err.negativeDate}'::character varying) AS exception;);

CREATE RULE "negativeExceptionUpdate" AS ON UPDATE TO "public"."event"
WHERE new.timewhen > new.timeuntil
DO (SELECT exception('${err.negativeDate}'::character varying) AS exception;);

CREATE OR REPLACE FUNCTION "public"."exception" (
  varchar
)
RETURNS boolean AS
$body$
BEGIN RAISE EXCEPTION '%',$1;END;
$body$
LANGUAGE 'plpgsql'
VOLATILE
CALLED ON NULL INPUT
SECURITY INVOKER
COST 100;