Stop asserting when there's not a descriptor for a relay being measured

When sbws doesn't have a descriptor for a relay that is being measured (but it has a network status document, and possibly the other way around), it raises and exception via this assert (https://github.com/torproject/sbws/blob/ee64d76df54ceb3a3c9e1e2a797fd70d68bb0035/sbws/lib/relaylist.py#L45), which causes the error callback in the thread pool in scanner. Instead, it should just return None (what can also be done by getattr and removes redundant code), and all the logic that depends on it will just work fine, it just won't be choose as an exit, since there's no information about the exit flags, but will get measured.