fix: Remove tiled writer if submitted task doesn't exist

Whenever a task is submitted, a TiledWriter is added to the run engine
and is only removed when the task is complete. If the task never starts
due to not being found, the writer would never be removed and subsequent
tasks would have duplicate writers causing write errors to tiled.

Catching the KeyError and removing the subscribers immediately is a
workaround that works but should be removed when the internal pending
tasks queue is removed.

Author: tpoliaw

src/blueapi/service/interface.py

@@ -221,7 +221,12 @@ def remove_callback_when_task_finished(
         )
 
     if task.task_id is not None:
-        active_worker.begin_task(task.task_id)
+        try:
+            active_worker.begin_task(task.task_id)
+        except KeyError:
+            active_worker.worker_events.unsubscribe(remove_callback)
+            active_context.run_engine.unsubscribe(tiled_writer_token)
+            raise
     return task