This replaces commit 53058090. Now
not only failure to start WU service, but also other errors during
MSU installation won't break installation. E.g., running WU service
as Guest does not prevent the service from starting; but installing
updates fail, which makes previous solution incomplete.

Instead, show a warning in those rare cases when an error happens,
and continue. It will allow users to see the reason if they see
"api-ms-win-*.dll missing" message later after installation. Of
course, some small percentage of these warnings will be false, e.g.
those on Windows 10. But those false messages must be really small
minority, because only when (1) the installation fails (2) on a
system with the component already present, such a message would be
false. And it will not prevent the installation.

This will not block unattended installations, since in those cases,
MsiProcessMessage() will do nothing.

Change-Id: I3a9e681e9d6701d092bd5c18bb4b68b4f77170f3
Reviewed-on: https://gerrit.libreoffice.org/64874
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>