Generate errors if targeting versions of .NET Core or .NET Standard that are higher than the current SDK supports

[dsplaisted]

As mentioned in https://github.com/dotnet/designs/issues/3, a .NET SDK will support targeting a maximum version of .NET Core and .NET Standard. If a project is opened that targets a higher version, an error message should be generated indicating that a newer SDK is needed.

For the 2.0 SDK, the versions should be capped at .NET Standard 2.0 and .NET Core 2.0.

Implementation-wise, we may want to derive the capped versions from the properties in BundledVersions.props.