BUG #14883: Syntax SQL error (42601), but should be a different error no

The following bug has been logged on the website:

Bug reference: 14883
Logged by: Stefan Hanenberg
Email address: stefan(dot)hanenberg(at)googlemail(dot)com
PostgreSQL version: 10.0
Operating system: Windows 10/64
Description:

I am not 100% sure whether this is really a bug, I report it anyway.

The following SQL statement leads to an error 42601 (syntax error), although
the error is not a syntax error.

create table t1 (a integer,b integer);
create table t2 (a integer,b integer,c integer);
select * from t1 union select * from t2;

We are currently running automated tests on hundreds of thousands SQL in
order to check, what kind of errors they contain. It is really problematic
for us that the error is a 42601 error, although it seems rather as if it is
a type error.