In PCbuild/openssl.props there is a task that deletes some copied files. 

This *might* fail on rebuilds where a Python process from a previous build has been running (for a very specific example, the rebuild as part of the release build triggered this).

We shouldn't need to fail the build in this case, so change the "TreatErrorsAsWarnings" attribute to "false"

(Note that this is not fixed by the KillPython step if the still-running executable is "python.exe" but we're building "python_d.exe" in the same folder. But warning and continuing is going to be fine for these particular files.)