Improve error message for checksum failures in pgstat_database.c

Improve error message for checksum failures in pgstat_database.c

This log message was referring to conflicts, but it is about checksum
failures.  The log message improved in this commit should never show up,
due to the fact that pgstat_prepare_report_checksum_failure() should
always be called before pgstat_report_checksum_failures_in_db(), with a
stats entry already created in the pgstats shared hash table.  The three
code paths able to report database-level checksum failures follow
already this requirement.

Oversight in b96d3c389755.

Author: Wang Peng <215722532@qq.com>
Discussion: https://postgr.es/m/tencent_9B6CD6D9D34AE28CDEADEC6188DB3BA1FE07@qq.com
Backpatch-through: 18