Normative: add Error.isError

spec.html

@@ -31212,6 +31212,16 @@ <h1>Properties of the Error Constructor</h1>
         <li>has the following properties:</li>
       </ul>
 
+      <emu-clause id="sec-error.iserror">
+        <h1>Error.isError ( _arg_ )</h1>
+        <p>This function performs the following steps when called:</p>
+        <emu-alg>
+          1. If _arg_ is not an Object, return *false*.
+          1. If _arg_ does not have an [[ErrorData]] internal slot, return *false*.
+          1. Return *true*.
+        </emu-alg>
+      </emu-clause>
+
       <emu-clause id="sec-error.prototype">
         <h1>Error.prototype</h1>
         <p>The initial value of `Error.prototype` is the Error prototype object.</p>