[8.8.2.2 Array methods] Enhance error message when using sum() without numeric data type (int or bit)

Author: LeeKaiXuan

src/PSSArrayInst.java

@@ -165,7 +165,7 @@ public PSSIntInst size() {
     public PSSIntInst sum() {
         if (!(m_elem_type_model instanceof PSSIntModel))
             PSSMessage.Error("ArrayInst",
-                    "Array.sum() is only used on Int type.");
+                    "Array.sum() is only used on `int` or `bit` type.");
 
         int sum = 0;
         for (PSSVal e : toVal().getValList())