Fixed .NET 4.0 detection in installer

Hi Jared, please see my peek at it in the pull request.

I've removed the old piece regarding MsiNetAssemblySupport (that doesn't really work since .NET 2.0) and adjusted the NETFRAMEWORK40CLIENT property condition + fixed a small typo in the error message.

I haven't actually tried to install it, however I'm fairly sure it should work and it compiles :)