Fetch data from the Redis cache.

def fetch(bank, key):
    '''
    Fetch data from the Redis cache.
    '''
    redis_server = _get_redis_server()
    redis_key = _get_key_redis_key(bank, key)
    redis_value = None
    try:
        redis_value = redis_server.get(redis_key)
    except (RedisConnectionError, RedisResponseError) as rerr:
        mesg = 'Cannot fetch the Redis cache key {rkey}: {rerr}'.format(rkey=redis_key,
                                                                        rerr=rerr)
        log.error(mesg)
        raise SaltCacheError(mesg)
    if redis_value is None:
        return {}
    return __context__['serial'].loads(redis_value)