Add numeric_int8_opt_error() to optionally suppress errors
This matches the existing numeric_int4_opt_error() (see commit
16d489b0fe). It will be used by a future JSON-related patch, which
wants to report errors in its own way and thus does not want the
internal functions to throw any error.
Author: Jeevan Chalke <jeevan.chalke@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/CAM2+6=XjTyqrrqHAOj80r0wVQxJSxc0iyib9bPC55uFO9VKatg@mail.gmail.com