Finding the number of rows affected by UPDATE or INSERT?

I am writing a bunch of stuff in pl/pgsql, and some of these functions
do UPDATEs or INSERTs on various tables.  How could I find out, from
within the pl/pgsql function, how many rows were affected?  This way,
if the update or insert fails for some reason, I can return a "failed"
message from the function, instead of pretending that everything is
ok.

Thanks