Exceptions on UI layer don't make the app crash
As we noticed with #173 (closed), when an exception is thrown in the AWT thread, for example when something goes wrong in response to a button click, it can happen that the UI crashes with an error dialog usually. While the UI is gone after confirming the dialog, it can happen that the briar process lives on. That's kind of bad because the app will still be online, receiving messages without the user noticing. Worse, it will probably not be possible to launch a new briar-desktop afterwards as the Tor-ports will be blocked.
I think it would be best to somehow hook into the graceful shutdown that already seems to exist for the UI somehow (a dialog is shown after all explaining the exception) and shut down the whole app.