Catching errors inside a function

I want to execute an INSERT query that may fail due to a primary key constraint
check. The primary key is on a field whose value is generated randomly by
another C function.
Is it possible to wrap the insert into a function that checks for failures and
retry the insert until success? Something like this:

function blah
{
   do {
        cod = generate code;
      execute insert with cod as primary key;
   } while (! errors);
}

I tried with SQL, PL/pgSQL and C functions but the backend aborts the
transaction and stops execution of the function as soon as the error is thrown.
Is there a way to disable this behaviour?

Thanks in advance for your help.

    Francesco Casadei