[wasm] Throw real Error on wasm trap

test/mjsunit/wasm/wasm-constants.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --expose-wasm
 
 function bytes() {
   var buffer = new ArrayBuffer(arguments.length);
   var view = new Uint8Array(buffer);
   for (var i = 0; i < arguments.length; i++) {
@@ skipping 272 matching lines @@
 function assertTraps(trap, code) {
     var threwException = true;
     try {
       if (typeof code === 'function') {
         code();
       } else {
         eval(code);
       }
       threwException = false;
     } catch (e) {
-      assertEquals("string", typeof e);
-      assertEquals(kTrapMsgs[trap], e);
+      assertEquals("object", typeof e);
+      assertEquals(kTrapMsgs[trap], e.message);
       // Success.
       return;
     }
     throw new MjsUnitAssertionError("Did not trap, expected: " + kTrapMsgs[trap]);
 }