ExcelDna.Integration contains no types and other errors after upgrade

We now target .NET 4.5+ in the NuGet package for v. 1.2
If a project is targeting an older version of .NET, e.g. .NET 4, then upgrading the package leads to confusing errors until the project is upgraded. Is there a way to give a more sensible message? For example, though maybe there is a better plan, make some library that targets .NET 4 where everything is stubbed out and marked obsolete.