Avoid an error in the fallback script when a fallback doesn't have any uptime

Sometimes, the fallback generation script doesn't add attributes to the
fallbacks in the list. If this happens, log an error, and avoid selecting
that fallback.

This is a rare issue: it should not change selection behaviour.

Fixes issue #20945.

changes/fallbacks-201612

@@ -33,3 +33,5 @@
       OnionOO. When a relay operator has multiple relays, this prioritises
       relays that are up over relays that are down.
       Closes ticket #20926; bugfix on 0.2.8.3-alpha.
+    - Stop failing when a relay has no uptime data in updateFallbackDirs.py.
+      Closes ticket 20945; bugfix on tor-0.2.8.1-alpha.

scripts/maint/updateFallbackDirs.py

@@ -862,6 +862,7 @@ class Candidate(object):
       self._badexit = self._avg_generic_history(badexit) / ONIONOO_SCALE_ONE
 
   def is_candidate(self):
+    try:
       if (MUST_BE_RUNNING_NOW and not self.is_running()):
         logging.info('%s not a candidate: not running now, unable to check ' +
                      'DirPort consensus download', self._fpr)
@@ -894,6 +895,10 @@ class Candidate(object):
           or self._data['consensus_weight'] < 1):
         logging.info('%s not a candidate: consensus weight invalid', self._fpr)
         return False
+    except BaseException as e:
+      logging.warning("Exception %s when checking if fallback is a candidate",
+                      str(e))
+      return False
     return True
 
   def is_in_whitelist(self, relaylist):