2 x duplicate key value violation: which exception comes first ?

Hello,

I've a table with 1 primary key and 1 unique index.

I would like to handle differently the duplicate key violations
depending on which constraints are affected.
(Within a plpgsql CATCH block, using SQLERRM).

It seems, that when both constraints would be violated, the exception is
always about the primary key.

Can I rely on that or should I expect the exception order being random ?

Thanks,

Marc Mamin