Do not create output files on error

[kim-fehl]

I used the wrong path to my database for RDPTools classify, and got the corresponding error:
Command Error: Unable to process file format UNKNOWN
However, empty output files still have been created.
I use Snakemake pipeline system, and if the output of the pipeline step is created, it thinks that everything is all right.
I would suggest either return error code to the system or at least do not create output files in such case, so the pipeline could understand that this step was broken.