daemon.run_gui: make sure to exit process on exception

previously, if GUI-related imports raised, the GUI would not start
but the process would not exit (e.g. asyncio event loop would go on)

Traceback (most recent call last):
  File "...\electrum\electrum\daemon.py", line 517, in run_gui
    gui = __import__('electrum.gui.' + gui_name, fromlist=['electrum'])
  File "...\electrum\electrum\gui\qt\__init__.py", line 39, in <module>
    from PyQt5.QtGui import QGuiApplication
ImportError: DLL load failed while importing QtGui: The specified module could not be found.

run_electrum

@@ -370,8 +370,13 @@ if __name__ == '__main__':
         if fd is not None:
             plugins = init_plugins(config, config.get('gui', 'qt'))
             d = daemon.Daemon(config, fd)
-            d.run_gui(config, plugins)
-            sys_exit(0)
+            try:
+                d.run_gui(config, plugins)
+            except BaseException as e:
+                _logger.exception('daemon.run_gui errored')
+                sys_exit(1)
+            else:
+                sys_exit(0)
         else:
             result = daemon.request(config, 'gui', (config_options,))