Do not eagerly convert exception to string when creating a message object

test/mjsunit/regress/regress-crbug-490680.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.
+
+var sentinel = null;
+
+try {
+  throw { toString: function() { sentinel = "observed"; } };
+} catch (e) {
+}
+
+L: try {
+  throw { toString: function() { sentinel = "observed"; } };
+} finally {
+  break L;
+}
+
+assertNull(sentinel);