Implement latest traceback global signal

[tfuxu]

Implement a custom signal that will send the latest traceback to HalftoneApplication or HalftoneMainWindow

[tfuxu]

I pivoted to storing latest error text in the ErrorPage widget. That way, unrecoverable tracebacks land straight in ErrorPage and less important errors will be stored as a state in error toasts (when I implement a stateful version of them).