class Foo:
    def _repr_mimebundle_(self, include, exclude):
        raise NotImplementedError("This error is ignored")

Foo()

The is due to a deliberate change introduced in #4832 (to resolve issue #4792).
The recommended solution was for the formatter functions to return None.

As ignoring NotImplementedErrors also ignores those generated outside the formatter functions, would it make sense to either:

1. revert the change, if the case that triggered it did switch to returning None.
2. or, inspect and only ignore the error if it is indeed generated in the body of a formatter function.