Obey handler.raise_error in _ahandle_event_for_handler (#6001)

Obey `handler.raise_error` in `_ahandle_event_for_handler`

Exceptions for async callbacks were only logged as warnings, also when
`raise_error = True`

#### Who can review?

  @hwchase17

   @agola11

langchain/callbacks/manager.py

@@ -238,6 +238,8 @@ async def _ahandle_event_for_handler(
         else:
             logger.warning(f"Error in {event_name} callback: {e}")
     except Exception as e:
+        if handler.raise_error:
+            raise e
         logger.warning(f"Error in {event_name} callback: {e}")