browser runner: check negative regex

1. use negative regex (instead of ".") to check iframeError

2. make Test262Error.prototype.toString() always include the string Test262Error (no spaces) so it matches /Test262Error/

3. modify check for supportsWindowOnerror - require that first argument (message) to onerror() contains the error name by checking for /Error:/.

Change (3) above forces IE11 onto the !supportsWindowOnerror path.

This means that Test262 tests are run inside an `eval("try{" + body + "}")` on IE11.  Errors are caught and explicitly have toString() called.  Without this, IE11 passes only the `message` property to onerror(), and regexes that expect to match error name fail.

Fixes #83