Added REMOVE_NOT_PERMITTED to ReviewerResult

Previously, RemoveReviewer would store error details in the `name`
property of ReviewerResult because there were not specific enough error
codes.  This change lets us keep the intended seperation of argument and
error detail.

Change-Id: I3eb431a017cd39e19276f98663f7da37f0d53f43

gerrit-gwtui/src/main/java/com/google/gerrit/client/changes/ChangeConstants.java

@@ -78,6 +78,8 @@ public interface ChangeConstants extends Constants {
 
   String approvalTableReviewer();
   String approvalTableAddReviewer();
+  String approvalTableRemoveNotPermitted();
+  String approvalTableCouldNotRemove();
 
   String changeInfoBlockOwner();
   String changeInfoBlockProject();