gen_node_support.pl: improve error message for unclosed struct.

gen_node_support.pl: improve error message for unclosed struct.

This error message was 'runaway "struct_name"', which isn't all
that clear; I think 'could not find closing brace for "struct_name"'
is better.  Also, provide the location of the struct start using the
script's usual '$file:$lineno' style.

Bug: #18901
Reported-by: Clemens Ruck <clemens.ruck@t-online.de>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/18901-424272abe01357e6@postgresql.org