Drop tables and functions only if exist
    
    At the time of first initialization of Koji database tables and
    functions don't exist. This isn't really a problem as SQL commands later
    in the script create them.
    
    For non-existent tables or functions psql prints NOTICE instead of an
    ERROR. Therefore one can check if database was initialized correctly
    with ON_ERROR_STOP set (see psql(1)):
    
         psql returns 3 if an error occurred in a script and the variable
         ON_ERROR_STOP was set.