[tor-bugs] #28870 [Core Tor/sbws]: Stop asserting when there's not a descriptor for a relay being measured

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#28870: Stop asserting when there's not a descriptor for a relay being measured
-------------------------------+-------------------------------
     Reporter:  juga           |      Owner:  (none)
         Type:  defect         |     Status:  new
     Priority:  Medium         |  Milestone:  sbws: 1.0.x-final
    Component:  Core Tor/sbws  |    Version:  sbws: 1.0.2
     Severity:  Normal         |   Keywords:
Actual Points:                 |  Parent ID:  #28663
       Points:                 |   Reviewer:
      Sponsor:                 |
-------------------------------+-------------------------------
 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.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28870>
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