fix(protocol-tests): ser test handler should throw error not object (smithy-lang#327)

Author: AllanZhengYP

smithy-typescript-codegen/src/main/resources/software/amazon/smithy/typescript/codegen/protocol-test-stub.ts

@@ -5,8 +5,10 @@ import { Readable } from 'stream';
 /**
  * Throws an expected exception that contains the serialized request.
  */
-class EXPECTED_REQUEST_SERIALIZATION_ERROR {
-    constructor(readonly request: HttpRequest) {}
+class EXPECTED_REQUEST_SERIALIZATION_ERROR extends Error {
+    constructor(readonly request: HttpRequest) {
+      super();
+    }
 }
 
 /**