Improve the error message in COPY with HEADER option.

Improve the error message in COPY with HEADER option.

The error message reported for invalid values of the HEADER option in COPY
command previously used the term "non-negative integer", which is
discouraged by the Error Message Style Guide because it is ambiguous about
whether zero is allowed.

This commit improves the error message by replacing "non-negative integer"
there with "an integer value greater than or equal to zero" to make
the accepted values explicit.

Author: Shinya Kato <shinya11.kato@gmail.com>
Reviewed-by: Alvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Steven Niu <niushiji@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwE86PcuPZbP=aurmW7Oo=eycF10gxjErWq4NmY-5TTX4Q@mail.gmail.com