mkdocs_gallery_expected_error code-block option to show an error

[smarie]

Sometimes we write a code block to show that something raises an error.
It is a bit tideous to have to try/catch this error to prevent mkdocs-gallery to fail.
It would be much better to be able to say that we expect it:

"""
Example
=======

The following code raises an error
"""
# mkdocs_gallery_expected_error
1 + "hello"

# %%
# But this works:

1 * "hello"

We could maybe have a mkdocs_gallery_expected_error similar to the mkdocs_gallery_defer_figures configuration.
It could even accept an optional argument (the exception type, as in pytest.raises)

Of course as always let's first ask the sphinx-gallery team to keep aligned.

Existing code-block options are listed at the bottom of https://sphinx-gallery.github.io/stable/configuration.html?highlight=sphinx_gallery_#list-of-config-options

See also https://sphinx-gallery.github.io/stable/configuration.html#dont-fail-exit which is related but does not fit the same purpose.