Best Practice when Encounter Invalid Stored Procedure Parameters

In c++, whenever we encounter an unexpected parameters, here is what we usually did :

bool fun(int i) {
if (i < 0) {
return false;
}
}

void fun(int i) {
if (i < 0) {
throw std::exception("Invalid parameter");
}
}

void fun(int i) {
assert (i >= 0);
}

How about stored procedure? Now, I have the following stored procedure :

CREATE OR REPLACE FUNCTION insert_unit(text[], text[])
RETURNS unit AS
$BODY$DECLARE
_measurement_types ALIAS FOR $1;
_measurement_units ALIAS FOR $2;
_unit unit;
BEGIN
IF array_upper(_measurement_values, 1) != array_upper(_measurement_units, 1) THEN
RAISE NOTICE 'What I should do here to return early???';
END IF;

May I know what is the good practice to handle invalid parameters? I am using libpq to interface with PostgreSQL.

Thanks and Regards
Yan Cheng CHEOK