All rst files are rebuilt if code cell has a Python error

[moorepants]

I currently edit my rst files in an editor and run sphinx autobuild to get continuous updates to a rendered HTML version. This works quite nicely and the sphinx cache ensures only the page your are editing is executed. But if you are working in a jupyter-execute cell and your code causes a Python error, then the sphinx build fails and seems to cause the entire cache to reset which then causes all rst files to be rebuilt. Once you have a large set of rst files that take significant execution time each, this breaks the usefulness of autobuild. It would be better if the code cell execution error did not trigger a sphinx cache reset.