Make consistent a couple of log messages when parsing HBA files

Make consistent a couple of log messages when parsing HBA files

This commit adjusts two log messages:
- When a field in pg_ident.conf is not populated, report the line of the
configuration file in an error context message instead of the main
entry.
- When parsing pg_ident.conf and finding an invalid regexp, add some
information about the line of the configuration file involved within an
error context message.

Author: Julien Rouhaud
Discussion: https://postgr.es/m/20220223045959.35ipdsvbxcstrhya@jrouhaud