Right way to reject INSERTs and UPDATEs

Hello!
I have postgresql table that stores dome date range: it has range-begin
and range-end columns of type date. I want to enforce that 1) range-begin
would always before range-end and 2) there would be no range overlaps.

First problem could be solved with CHECK constraint. The only possibility
to solve second problem is to use a trigger. Trigger would fire
BEFORE INSERT OR UPDATE and FOR EACH ROW and invoke pgplsql function
that returns NULL or "new" depending on condition met or not;

These solution work and enforce data integrity as needed. Sadly, database
interface I use (Trolltech Qt 3) can't track when trigger aborts
update or insert. Can I raise exception in trigger or something?

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