Throw better exception when refs dir is missing

Author: chuseman

src/GitVersionCore/GitVersionCacheKeyFactory.cs

@@ -42,7 +42,7 @@ static List<string> CalculateDirectoryContents(string root)
 
             if (!Directory.Exists(root))
             {
-                throw new ArgumentException();
+                throw new DirectoryNotFoundException($"Root directory does not exist: {root}");
             }
 
             dirs.Push(root);