bug: confusing err when parsing ancient timestamptz

[xiangjinwu]

create table test (
  tz timestamptz
);

insert into test values ('0001-01-01 00:00:00Z');

will cause Parse error: Can't cast string to timestamp (expected format is YYYY-MM-DD HH:MM:SS[.D+{up to 6 digits}] or YYYY-MM-DD HH:MM or YYYY-MM-DD or ISO 8601 format).
'1969-01-01 00:00:00Z' is ok.

Originally posted by @xuefengze in #13496 (comment)

[xiangjinwu]

Not useful in practice, but common when people tries to test edge cases.