HPCC-9208 Correctly catch errors returned by pssh and prevent core

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4315 from ghalliday/issue9208

HPCC-9208 Correctly catch errors returned by pssh and prevent core

Reviewed-By: Attila Vamos <attila.vamos@lexisnexis.com>
Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>