Cause plpgsql's PERFORM to behave according to its documentation,

which says that PERFORM will execute any SELECT query and discard the
result.  The former implementation would in fact raise an error if the
result contained more than one row or more than one column.

Also, change plpgsql's error-logging mechanism to emit the additional
messages about error location at NOTICE rather than DEBUG level.  This
allows them to be seen by the client without having to dig into the
postmaster log file (which may be nonexistent or inaccessible by the
client).