Checking if Aggregate exists

Hi,

I have an upgrade script that is supposed to install items into a
postgresql database if they don't already exist.  One of the items I'm
having a hard time with is aggregates.  I want to check if aggregate
foo doesn't exist, then run an SQL command to generate it.

if (check_aggregate_exists('foo') === false) {
	$db->execute("CREATE AGGREGATE foo ....");
}

My code to check if an aggregate exists runs this query:

SELECT * FROM pg_catalog.pg_aggretate WHERE aggfnoid = 'foo'::REGPROC;

That works great IF foo exists, problem is if foo doesn't exist when
this query runs I get an error:

ERROR:  function "foo" does not exist

The error causes my transaction to abort and rollback all the changes
I had already made in my update script. 

Is there a better way to do this?  Maybe a stored proc that eats the
error message?

Thanks,

Josh