HPCC-12573 Report a more helpful error message running an uncompiled wu

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #6641 from ghalliday/issue12573

HPCC-12573 Report a more helpful error message running an uncompiled wu

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>