Make mesage at end-of-recovery less scary.

Hello, this is a followup thread of [1].

# I didn't noticed that the thread didn't cover -hackers..

When recovery of any type ends, we see several kinds of error messages
that says "WAL is broken".

> LOG: invalid record length at 0/7CB6BC8: wanted 24, got 0
> LOG: redo is not required
> LOG: database system is ready to accept connections

This patch reduces the scariness of such messages as the follows.

> LOG: rached end of WAL at 0/1551048 on timeline 1 in pg_wal during crash recovery
> DETAIL: invalid record length at 0/1551048: wanted 24, got 0
> LOG: redo is not required
> LOG: database system is ready to accept connections

[1] https://www.postgresql.org/message-id/20200117.172655.1384889922565817808.horikyota.ntt%40gmail.com

I'll register this to the coming CF.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center