sphinx-needs: needs_warnings not returning error code or writing to error log

Usage of -W shall handle all warnings as errors and the sphinx-build executable shall return a non-zero value as error code. Also the usage of -w <file> shall write all warnings to a file.

This works incase of Sphinx warnings, but not for warnings created with needs_warnings. Because of this pipelines cannot be configured to break incase of needs_warnings.

To reproduce the problem, do the following steps

1. Create needs_warnings in conf.py
2. Create a need warning within any rST
3. Execute sphinx-build as follows from a console

sphinx-build -M html docs _build -W --keep-going -w error.log  

1. Observe need_warnings on the console
2. Execute the following from the console

echo $?

1. 0 is returned (a non-zero value is expected)
2. Check the file error.log
3. Only the following line is present

WARNING: Sphinx-Needs warnings were raised. See console / log output for details.

All needs_warnings are expected in the log.

Could it be that needs_warnings is always writing to stdout instead of stderror (even with -W)?