Add HINT for COPY TO when WHERE clause is used.

Add HINT for COPY TO when WHERE clause is used.

COPY TO does not support a WHERE clause, and currently fails with the error:

    ERROR:  WHERE clause not allowed with COPY TO

Since the intended behavior can be achieved by using
COPY (SELECT ... WHERE ...) TO, this commit adds a HINT
to the error message:

    HINT:  Try the COPY (SELECT ... WHERE ...) TO variant.

This makes the error more informative and helps users
quickly find the alternative usage.

Author: Atsushi Torikoshi <[email protected]>
Reviewed-by: Jim Jones <[email protected]>
Discussion: https://postgr.es/m/3520c224c5ffac0113aef84a9179f37e@oss.nttdata.com