[Bug] Error "No .net API detected" is unclear and wastes many hours

**Describe the bug**

If all C# code classes are filtered out because of rules or the `filterConfig.yml`, an error is shown `No .net API detected` - this happens here: https://github.com/dotnet/docfx/blob/dd69466fa8f20771d759fe60b2260da0591c12f2/src/Docfx.Dotnet/DotnetApiCatalog.ManagedReference.cs#L41

I wasted at least 3 hours trying to fix this, because I assumed something was wrong with the `.csproj`. It took me forover to guess that my `filterConfig.yml` is wrong.

**Expected behavior**

Please improve the error message to something like this:

$"No .NET API detected (possibly due to filtering rules in filterConfig.yml) for {value}."

**Context (please complete the following information):**

- OS: Windows
- Docfx version: [e.g. 2.78.3]

**Additional context**

Add any other context about the problem here.