HealthService improvements. New method addException, support for stack trace and global error handler improvements available on Firefox 31.
Implementing retrival of alert text for onBeforeUnload dialogs.

Fixes issue #7901.
Revert "Bump the version of webbit to 0.4.15"

This reverts commit 41b8f0598546ac78d06eb3b39fec23d9917340b2.

Version 0.4.15 has an issue https://github.com/webbit/webbit/issues/134 that causes some selenium tests to fail, in particular ReferrerTest is broken.