Which record causes referential integrity violation on delete

In Postgres 8 I tried commad

DELETE FROM customer WHERE id=123

but got an error

ERROR: update or delete on "customer" violates foreign key constraint
"invoice_customer_fkey" on "invoice"'

How to determine the primary key of invoice table which causes this error
in generic way ?

Why Postgres does not report primary key value in error message ?

I it is not possible to add more information to error I need function which
takes 3 arguments:

table name ('customer')
field name ('id')
field value (123)

and returns the name of the table and primary key value which blocks
record deletion.

I looked to the ODBC log, but this does'nt contain more information
about error !?

Andrus.