Clean up gerrit_util's handling of unknown reviewer/cc

This is still important, but at least now it won't result
in a stacktrace.

Change-Id: If6e7e15a98b0a03df7e978201762fea670312ede
Reviewed-on: https://chromium-review.googlesource.com/408660
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index 9833747..da568e5 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -2842,8 +2842,10 @@
     if change_desc.get_cced():
       cc.extend(change_desc.get_cced())
     if cc:
-      gerrit_util.AddReviewers(
+      errors = gerrit_util.AddReviewers(
           self._GetGerritHost(), self.GetIssue(), cc, is_reviewer=False)
+      if errors:
+        return 1
 
     return 0