Fix error message when performing an operation on a keyword

lib/less/tree/operation.js

@@ -17,6 +17,11 @@ tree.Operation.prototype.eval = function (env) {
                     message: "Can't substract or divide a color from a number" };
         }
     }
+    if (!a.operate) {
+        throw { name: "OperationError",
+                message: "Operation on an invalid type" };
+    }
+
     return a.operate(this.op, b);
 };