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.
Test262 tests are run inside an eval() on IE11, and 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.