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