Allow samples.json to have extra sample infos

[ionox0]

If there are samples in the samples.json file that didn't get run through the qc generation workflow, the general_parse_stats.py file will exit due to these lines:

https://github.com/msk-access/access_utils/blob/multiqc-utils/multiqc_prep/general_stats_parse.py#L322-L327

It may be better to change sys.exit(1) to a continue, so that this script can handle parsing just the subset of samples that overlap

Similarly, a return could be used here to make the module more flexible

    if not os.path.exists(args.samples_json):
        print('could not find file: {}!'.format(args.samples_json))
        sys.exit(1)