added missing nuspec.xsd namespace and better exception

This is a bug found when using `0.9.1.` It is missing `http://schemas.microsoft.com/packaging/2011/10/nuspec.xsd`. I also added an better message if another xsd is missing.


In Paket.Tests.fsproj below, the only change to the files is adding this:
```
    <Content Include="TestFiles\FSharp.Data.SqlEnumProvider.nuspec">
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </Content>
```
I'm not sure what is up with the rest Visual Studio 2013 switching the Choose & ItemGroup order. I tried twice with the same result.