Return a list of paths matching a pattern (or None on error).

def process_pattern(fn):
    """Return a list of paths matching a pattern (or None on error).
    """
    directory, pattern = validate_pattern(fn)
    if directory is not None:
        filenames = fnmatch.filter(auto(listdir, directory), pattern)
        if filenames:
            return [directory + '/' + sfn for sfn in filenames]
        else:
            print_err("cannot access '{}': No such file or directory".format(fn))