Make set_score silently fail if adj not in debate

This makes `VotingDebateResult.set_score(adjudicator, *args)` fail
silently if there isn't a scoresheet for `adjudicator`, which
(should) happen if the adjudicator isn't assigned to that debate.
It logs an exception, so the failure isn't completely silent, but
it doesn't alert the user immediately.

Relates to #922. I'm not yet sure if making this silently fail
might cause problems later.

tabbycat/results/result.py

@@ -540,10 +540,9 @@ def set_score(self, adjudicator, side, position, score):
         try:
             scoresheet = self.scoresheets[adjudicator]
         except KeyError:
-            logger.exception("Adjudicator %s has a scoresheet but is not assigned \
-                to this debate. This debate's panel has probably been changed since \
-                this ballot was loaded for entry. Best to return to the results page \
-                and re-enter this ballot from scratch.", adjudicator.name)
+            logger.exception("Tried to set score by adjudicator %s, but this adjudicator "
+                "doesn't have a scoresheet.", adjudicator)
+            return
         try:
             scoresheet.set_score(side, position, score)
         except AttributeError: