tools: ynl-gen: re-raise the exception instead of printing

traceback.print_exception() seems tricky to call, we're missing
some argument, so re-raise instead.

Reported-by: Chuck Lever III <chuck.lever@oracle.com>
Fixes: 3aacf828

 ("tools: ynl: add an object hierarchy to represent parsed spec")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>