If an developer attempts to "subclass" Error by running
`MyError.prototype = new Error();`, then the internal v8::Message object
that's produced and handed off to `window.onerror` handlers is poorly
stringified as "[object Object]".

This patch adjusts the stringification process for these objects to
include not only native Error objects, but also objects that have Error
in their prototype chain, and haven't overwritten Error.toString with
some custom variant.

BUG=2822
R=mstarzinger@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/21761002

Patch from Mike West <mkwst@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16075 ce2b1a6d-e550-0410-aec6-3dcde31c8c00