Correct usage of FOR UPDATE?

Hello,
I have a simple table-based queue system, and I'd looking for some advice on
improving my dequeue function. it boils down to:

SELECT id
FROM queue
WHERE <whereclause>
FOR UPDATE NOWAIT;

which works well, in that no item gets dequeued more that once. The issue,
however is that when a contention occurs, the error "could not obtain lock
on row in relation" is raised.

Is there a way that I can rewrite this so that when a contention occurs,
there is no error? I would like the "winning" process to lock the row, and
the "losing" process to select null rather than raise the rowlock error.

thanks beforehand for any help!