During restore, generate an error if .NET 4.6.1 support for .NET Standard 2.0 is required but not installed

[dsplaisted]

Support for .NET Standard 2.0 in .NET Framework 4.6.1 is going to ship for VS 2015 as a separate MSI. During NuGet restore, if a project depends on any .NET Standard 2.0 assets, then NuGet should check to see whether the .NET Standard 2.0 support is installed, and if not, generate an error.

This is a piece of the work required for dotnet/sdk#1267, which tracks the overall work.

[rrelyea]

We need this error to be checked to work in VS2015 and NuGet.exe code paths.

We need to understand what to check for. Can you folks (@ericstj @dsplaisted) tell us that?

[rrelyea]

This is not quite "4.3" work. But very soon thereafter targeting VS2015. Likely "3.6.0"

[jainaashish]

@dsplaisted @livarcocc @ericstj @nguerrera We're picking this workitem now for 15.4 so can someone tell us what we need to check for here in order to figure out support for Net standard 2.0 for VS2015? and also the content for the error message?