Fix keyerror in presubmit_support

BUG=669153

Change-Id: I1e078bfc83ccfa4fd3785fd7388560c4ba43b735
Reviewed-on: https://chromium-review.googlesource.com/416389
Reviewed-by: Mike Stipicevic <stip@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

presubmit_support.py

@@ -253,7 +253,7 @@ class GerritAccessor(object):
   def GetChangeReviewers(self, issue, approving_only=True):
     cr = self.GetChangeInfo(issue)['labels']['Code-Review']
     max_value = max(int(k) for k in cr['values'].keys())
-    return [r['email'] for r in cr['all']
+    return [r.get('email') for r in cr.get('all', [])
             if not approving_only or r.get('value', 0) == max_value]