Call to set_exception_handler ignores previously set callbacks

[mterwill]

My app leverages a custom exception handler. It is registered early on, before the bugsnag handler. When I register the bugsnag handler, it overwrites my custom exception handler.

Because set_exception_handler returns the previously set callback, it is possible to create a nice stack of these exception handling callbacks. It would be nice if Bugsnag\Handler::register respected this pattern. One example implementation of the expected behavior is Monolog\ErrorHandler.

set_error_handler also returns the previously set callback.