propagate the error when DASQuery parsing fails

fixes issue #4107 

without this patch, the user would get meaningless error msg "argument of type 'NoneType' is not iterable", caused by unhandled parser error ( [ply2mongo(mongo_query)](https://github.com/dmwm/DAS/blob/68bd90430544b642ae302977081997e6d2200823/src/python/DAS/core/das_ply.py#L478)  expects a dict, but gets ```None```).