Gracefully handling variable-length data types at the 8k limit?

I sent this to general:

I have written an aggregate function which returns a "varlena" type whose
length is proportional to the number of rows passed to the function.

My question is what is the consensus on the best way to handle the case
where the input to the aggregate causes the length of the variable to
exceed the 8k limit? Abort the query? Generate an error message, and
discard subsequent data? Silently discard data? Something else?

Apparently, not only is there no consensus, but nobody seems to have any
opinions either. So, I would like to solicit opinions.

What is the proper thing for aggregate functions and operators to do when
they discover they need more than 8k for a variable length data type?

-Michael Robinson