BUG #14861: Handle syntax_error

The following bug has been logged on the website:

Bug reference: 14861
Logged by: Andrey Shvidky
Email address: andrey_shvidky(at)hotmail(dot)com
PostgreSQL version: 9.6.5
Operating system: Windows 7 x64
Description:

/*
Faced with such a unfairness.
I have a do block or function with error handling block.
In case when error type is 42601 (syntax_error) I can't get correct error
context.
The error context contains information about do block or function itself,
but not about last executed instruction.
In the folowing example, if I remove error handling I will happily have such
a message:

ERROR: subquery must return only one column
LINE 1: SELECT (select id, val1 from aaa)

But in case when error handling presents I have only sad:

message_text = subquery must return only one column
pg_exception_context = PL/pgSQL function inline_code_block line 9 at
assignment
pg_exception_detail =

Where information about "SELECT (select id, val1 from aaa)" hided?

Repro script:
*/

create temp table aaa (id int, val1 text);

do language plpgsql $$
declare
error_message text;
error_detailed_message text;
error_context text;
arr_test aaa[];
begin
--arr_test = (select row(id, val1) from aaa); -- No error, correct call
arr_test = (select id, val1 from aaa); -- Error 42601 syntax_error
exception
--when syntax_error then
when others then
get stacked diagnostics
error_message = message_text
,error_context = pg_exception_context
,error_detailed_message = pg_exception_detail;

raise notice 'message_text = %', error_message;
raise notice 'pg_exception_context = %', error_context;
raise notice 'pg_exception_detail = %', error_detailed_message;
end
$$;

drop table if exists aaa;