Server stops rebuilding on changes if the pre-build step fails

[pradyunsg]

I think the title is a sufficiently clear description. Let me know if more details are necessary.

How to reproduce

1. Checkout https://github.com/pradyunsg/furo/tree/2024.04.27

2. Run nox -s docs-live

3. Wait for the initial build to complete.

4. Modify source tree such that the pre-build command would fail.

   For this reproducer, modify src/furo/assets/styles/content/_admonitions.sass to be invalid by adding a comma at the end of line 3 of that file.

5. Notice an exception originating from the pre-build command and the output statement from sphinx-autobuild stating:

   Pre-build command exited with exit code: 3
   Please fix the cause of the error above or press Ctrl+C to stop the server.
   

6. Fix the pre-build command and save a file that would be watched by the server.

   For this reproducer, undo the addition done in step 4 (i.e. remove the comma) and save.

7. Notice that no rebuild was triggered.

The server keeps serving the relevant changes while this is happening.