missing warning in pg_import_system_collations

Hello hackers,

In pg_import_system_collations() there is this fragment of code:

enc = pg_get_encoding_from_locale(localebuf, false);
if (enc < 0)
{
    /* error message printed by pg_get_encoding_from_locale() */
    continue;
}

However, false passed to pg_get_encoding_from_locale() means 
write_message argument is false, so no error message is ever printed.
I propose an obvious patch (see attachment).

Introduced in aa17c06fb in January 2017 when debug was replaced by 
false, so I guess back-patching through 10 would be appropriate.

-- 
Anton Voloshin
Postgres Professional, The Russian Postgres Company
https://postgrespro.ru