Fix error message when adding a reviewer that does not exist

If the name given for a reviewer does not match to a user the
code falls into trying to match a group. If it also does not
match a group, report that neither is found instead of saying
"Group Not Found:user@example.com".

Change-Id: I09fd4448508430faa4fdd893687f7a0def2e1141

gerrit-server/src/main/java/com/google/gerrit/server/change/PostReviewers.java

@@ -129,7 +129,13 @@ public class PostReviewers implements RestModifyView<ChangeResource, Input> {
       Account.Id accountId = accounts.parse(input.reviewer).getAccountId();
       return putAccount(reviewerFactory.create(rsrc, accountId));
     } catch (UnprocessableEntityException e) {
-      return putGroup(rsrc, input);
+      try {
+        return putGroup(rsrc, input);
+      } catch (UnprocessableEntityException e2) {
+        throw new UnprocessableEntityException(MessageFormat.format(
+            ChangeMessages.get().reviewerNotFound,
+            input.reviewer));
+      }
     }
   }