Handle exceptions in scheduler and main threads

Unhandled exceptions in the scheduler thread should no longer cause
scheduled tasks to stop executing.

We also improve both loops by cleanly handling BaseExceptions such as
KeyboardInterrupt and SystemExit

mmpy_bot/threadpool.py

@@ -64,6 +64,9 @@ def handle_work(self):
                 function(*arguments)
             except Exception:
                 log.exception("Unhandled exception in main loop")
+            except BaseException:
+                # Can be KeyboardInterrupt, SystemExit, ...
+                self.alive = False
             # Notify the pool that we finished working
             self._queue.task_done()
             self._busy_workers.get()
@@ -73,7 +76,13 @@ def run_pending():
             log.info("Scheduler thread started.")
             while self.alive:
                 time.sleep(trigger_period)
-                default_scheduler.run_pending()
+                try:
+                    default_scheduler.run_pending()
+                except Exception:
+                    log.exception("Unhandled exception in main loop")
+                except BaseException:
+                    # Can be KeyboardInterrupt, SystemExit, ...
+                    self.alive = False
             log.info("Scheduler thread stopped.")
 
         self.add_task(run_pending)