Better error messages when specifying wrong/non-existent framework

[blackdwarf]

Repro

1. Run dotnet new
2. Run dotnet restore
3. Run dotnet build -f net461

Expected result

An error saying that the TFM is invalid because it doesn't exist in my project.

Actual result

I get an error about having to put in RIDs. However, if I replace the repro command with dotnet build -f netcoreapp3.0 I will get a red wall of build errors because the ref assemblies cannot be found (which is normal, because they don't exist).

It would be much better if we could provide a good error message here.