Try to improve async stack traces for exceptions in tasks (#1442)

* Try to improve async stack traces for exceptions in tasks

* Lint

libs/langgraph/langgraph/pregel/executor.py

@@ -120,14 +120,14 @@ class AsyncBackgroundExecutor(AsyncContextManager):
 
     def done(self, task: asyncio.Task) -> None:
         try:
-            task.result()
-        except GraphInterrupt:
-            # This exception is an interruption signal, not an error
-            # so we don't want to re-raise it on exit
-            self.tasks.pop(task)
-        except BaseException:
-            pass
-        else:
+            if exc := task.exception():
+                # This exception is an interruption signal, not an error
+                # so we don't want to re-raise it on exit
+                if isinstance(exc, GraphInterrupt):
+                    self.tasks.pop(task)
+            else:
+                self.tasks.pop(task)
+        except asyncio.CancelledError:
             self.tasks.pop(task)
 
     async def __aenter__(self) -> Submit:
@@ -146,12 +146,13 @@ class AsyncBackgroundExecutor(AsyncContextManager):
         # wait for all tasks to finish
         if self.tasks:
             await asyncio.wait(self.tasks)
-        # re-raise the first exception that occurred in a task
+        # if there's already an exception being raised, don't raise another one
         if exc_type is None:
-            # if there's already an exception being raised, don't raise another one
+            # re-raise the first exception that occurred in a task
             for task in self.tasks:
                 try:
-                    task.result()
+                    if exc := task.exception():
+                        raise exc
                 except asyncio.CancelledError:
                     pass