Fix crash on exit if any plugins have loaded code that raises exceptions during shutdown. Fixes #1592414 [Crash on quit (v2.57.1, OS X El Capitan)](https://bugs.launchpad.net/calibre/+bug/1592414)

src/calibre/gui2/ui.py

@@ -941,7 +941,15 @@ class Main(MainWindow, MainWindowMixin, DeviceMixin, EmailMixin,  # {{{
             pass
         self.hide_windows()
         # Do not report any errors that happen after the shutdown
-        sys.excepthook = sys.__excepthook__
+        # We cannot restore the original excepthook as that causes PyQt to
+        # call abort() on unhandled exceptions
+        import traceback
+        def eh(t, v, tb):
+            try:
+                traceback.print_exception(t, v, tb, file=sys.stderr)
+            except:
+                pass
+        sys.excepthook = eh
         if self._spare_pool is not None:
             self._spare_pool.shutdown()
         from calibre.db.delete_service import shutdown