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 <torikoshia@oss.nttdata.com>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Discussion: https://postgr.es/m/3520c224c5ffac0113aef84a9179f37e@oss.nttdata.com

src/backend/parser/gram.y

@@ -3442,6 +3442,7 @@ CopyStmt:	COPY opt_binary qualified_name opt_column_list
 						ereport(ERROR,
 								(errcode(ERRCODE_SYNTAX_ERROR),
 								 errmsg("WHERE clause not allowed with COPY TO"),
+								 errhint("Try the COPY (SELECT ... WHERE ...) TO variant."),
 								 parser_errposition(@11)));
 
 					n->options = NIL;

src/test/regress/expected/copy2.out

@@ -163,6 +163,7 @@ COPY x TO stdout WHERE a = 1;
 ERROR:  WHERE clause not allowed with COPY TO
 LINE 1: COPY x TO stdout WHERE a = 1;
                          ^
+HINT:  Try the COPY (SELECT ... WHERE ...) TO variant.
 COPY x from stdin WHERE a = 50004;
 COPY x from stdin WHERE a > 60003;
 COPY x from stdin WHERE f > 60003;