Notebooks failing in readthedocs build does not trigger red status

[AdamGleave]

Bug description

Running notebooks is currently failing in the readthedocs build but as a warning only so CI wrongly looks green.

Steps to reproduce

From the logs, there are a variety of failures that are warnings only:

/home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/docs/tutorials/3_train_gail.ipynb: WARNING: Executing notebook failed: CellTimeoutError [mystnb.exec]
/home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/docs/tutorials/3_train_gail.ipynb: WARNING: Notebook exception traceback saved in: /home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/_readthedocs/html/reports/tutorials/3_train_gail.err.log [mystnb.exec]
/home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/docs/tutorials/4_train_airl.ipynb: WARNING: Executing notebook failed: CellExecutionError [mystnb.exec]
/home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/docs/tutorials/4_train_airl.ipynb: WARNING: Notebook exception traceback saved in: /home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/_readthedocs/html/reports/tutorials/4_train_airl.err.log [mystnb.exec]
/home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/docs/tutorials/5a_train_preference_comparisons_with_cnn.ipynb: WARNING: Executing notebook failed: CellExecutionError [mystnb.exec]
/home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/docs/tutorials/5a_train_preference_comparisons_with_cnn.ipynb: WARNING: Notebook exception traceback saved in: /home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/_readthedocs/html/reports/tutorials/5a_train_preference_comparisons_with_cnn.err.log [mystnb.exec]
/home/docs/checkouts/readthedocs.org/user_builds/imitation/checkouts/latest/docs/tutorials/8a_train_sqil_sac.ipynb: WARNING: Executing notebook failed: CellExecutionError [mystnb.exec]

Suggest we either make these warnings fatal, or unskip testing the notebooks in tests/test_examples.py so we test them on Linux again.

[AdamGleave]

@ernestum can you investigate this -- I think you have the most context on this as you made the original change to run notebooks as part of docs build in #565

This is a priority to fix as it is making the CI unreliable.

[ernestum]

I'm on it (#803). I hope a simple config change will do the trick.