Improve the log message output of basic_archive when basic_archive.archive_directory parameter is not set

Hi,

When archive_library is set to 'basic_archive' but
basic_archive.archive_directory is not set, WAL archiving doesn't work
and only the following warning message is logged.

$ emacs $PGDATA/postgresql.conf
archive_mode = on
archive_library = 'basic_archive'

$ bin/pg_ctl -D $PGDATA restart
....
WARNING: archive_mode enabled, yet archiving is not configured

The issue here is that this warning message doesn't suggest any hint
regarding the cause of WAL archiving failure. In other words, I think
that the log message in this case should report that WAL archiving
failed because basic_archive.archive_directory is not set. Thus, I think
it's worth implementing new patch that improves that warning message,
and here is the patch for that.

Best regards,
Tung Nguyen