Instead, print a message regarding the failure, pointing to new
`disable_failing_post_execute` trait for skipping failing callbacks.
When this flag is False (the default), failing callbacks will continue to be called.
When True, behavior is unchanged from previous, where callbacks are only allowed to fail once.
Invalid matplotlib data may raise inside print_figure. flush_figures()
is a post-exec function, so user errors should not raise. Instead,
call get_ipython().showtraceback() if called from IPython, raising as before