Disallow specifying ON_ERROR option without value.

Disallow specifying ON_ERROR option without value.

The ON_ERROR option of the COPY command previously allowed omitting
its value, which was inconsistent with the syntax synopsis in the
documentation and the behavior of other non-boolean COPY options.

This change enforces providing a value for the ON_ERROR option,
ensuring consistency across other non-boolean options and aligning
with the documented syntax.

Author: Atsushi Torikoshi
Reviewed-by: Masahiko Sawada
Discussion: https://postgr.es/m/a9770bf57646d90dedc3d54cf32634b2%40oss.nttdata.com