Don't fail builds when NuGet packages have already been pushed

[tomasaschan]

Currently the build fails if there is already a version pushed with the specific package number. This happens every now and then if a tagged commit is built more than once, for whatever reason; see e.g. https://ci.appveyor.com/project/tlycken/rdbmseventstore/build/0.6.0.build.224.

Would be nice if that didn't fail the badge on the readme... :/