HPCC-10365 Regression suite requires non-existent directory

Fix directory checking

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>
HPCC-10368 regress should give more information on syntax

Improve built-in help and usage hints.

Signed-off-by: Attila Vamos <attila.vamos@gmail.com>