How to use the crds.CrdsLookupError function in crds

import crds
    from crds import CrdsLookupError

    if download:
        try:
            reffile_mapping = crds.getreferences(parameter_dict, reftypes=reffile_types)
        except CrdsLookupError as e:
            raise ValueError("ERROR: CRDSLookupError when trying to find reference files for parameters: {}".format(parameter_dict))
    else:
        # If the files will not be downloaded, still return the same local
        # paths that are returned when the files are downloaded. Note that
        # this follows the directory structure currently assumed by CRDS.
        crds_path = os.environ.get('CRDS_PATH')
        try:
            reffile_mapping = crds.getrecommendations(parameter_dict, reftypes=reffile_types)
        except CrdsLookupError as e:
            raise ValueError("ERROR: CRDSLookupError when trying to find reference files for parameters: {}".format(parameter_dict))

        for key, value in reffile_mapping.items():
            # Check for NOT FOUND must be done here because the following
            # line will raise an exception if NOT FOUND is present
            if "NOT FOUND" in value:
                reffile_mapping[key] = "NOT FOUND"
            else:
                instrument = value.split('_')[1]
                reffile_mapping[key] = os.path.join(crds_path, 'references/jwst', instrument, value)

    return reffile_mapping