[wasm] Throw real Error on wasm trap

test/mjsunit/wasm/unreachable.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
 
 load("test/mjsunit/wasm/wasm-constants.js");
 load("test/mjsunit/wasm/wasm-module-builder.js");
 
 var main = (function () {
   var builder = new WasmModuleBuilder();
   builder.addFunction("main", [kAstStmt])
     .addBody([kExprUnreachable])
     .exportAs("main");
 
   return builder.instantiate().exports.main;
 })();
 
 var exception = "";
 try {
     assertEquals(0, main());
 } catch(e) {
     print("correctly caught: " + e);
     exception = e;
 }
-assertEquals("unreachable", exception);
+assertEquals("unreachable", exception.message);