Passing `-Werror` to `pytest --nbmake` has no effect

[garrison]

Describe the bug
Passing -Werror to pytest --nbmake has no effect. It should, instead, cause the tests to fail if the code in a notebook causes a warning.

To Reproduce
Steps to reproduce the behavior:

1. Create a notebook which emits a warning.
2. Run it using pytest --nbmake --Werror **/*ipynb

Expected behavior
I expect tests to fail.

[alex-treebeard]

hi @garrison, thanks for this report. We generally intend to follow conventions in pytest so I'll mark this one as a bug.