behave: context.failed is a read-only attribute

[nicholasyang2022]

The step functions in behave test try to indicate failures by setting context.failed to True. This does work in the expected way, because context.failed is a read-only attribute[1]. A failure should be indicated by throwing an exception.

[1] https://behave.readthedocs.io/en/latest/context_attributes.html?highlight=context#behave-attributes