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b934677a - Obey handler.raise_error in _ahandle_event_for_handler (#6001)

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