Again improve error message of system time rewind.
Again improve error message of system time rewind.

The previous commit is not correct because this error
message can be shown in archived WAL file or server log
file backup. So, change the hint message more generally.

In addition, this patch changes error level from ERROR
to FATAL. This is because this cannot be resolved only
by pg_rman. If this error can be shown in archived WAL
file backup, the full backup mode is not a solution.
Users has to check the file and modify the timestamp
in order to run full backup successfully.

ooyamams/pg_rman,ooyamams/pg_rman