Make some xlogreader messages more accurate

Make some xlogreader messages more accurate

When you have some invalid WAL, you often get a message like "wanted
24, got 0".  This is a bit incorrect, since it really wanted *at
least* 24, not exactly 24.  This updates the messages to that effect,
and also adds that detail to one message where it was available but
not printed.

Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Reviewed-by: Jeevan Ladhe <jeevanladhe.os@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/726d782b-5e45-0c3e-d775-6686afe9aa83%40enterprisedb.com