Added exception for non-existent directory in ImageLoader (dotnet#4831)

* Added exception for non-existant directory

* Added comments

* Update ImageLoader.cs

Author: mstfbl

src/Microsoft.ML.ImageAnalytics/ImageLoader.cs

@@ -103,9 +103,12 @@ internal ImageLoadingTransformer(IHostEnvironment env, string imageFolder = null
         internal ImageLoadingTransformer(IHostEnvironment env, string imageFolder = null, bool type = true, params (string outputColumnName, string inputColumnName)[] columns)
             : base(Contracts.CheckRef(env, nameof(env)).Register(nameof(ImageLoadingTransformer)), columns)
         {
-            // Throws ArgumentException if given imageFolder path is invalid or empty. Note: imageFolder may be null in this case.
+            // Throws ArgumentException  if given imageFolder path is invalid or empty. Note: imageFolder may be null in this case.
             if (imageFolder != null)
-                ImageFolder = Path.GetFullPath(imageFolder);
+                if (Directory.Exists(imageFolder))
+                    ImageFolder = Path.GetFullPath(imageFolder);
+                else
+                    throw new ArgumentException(String.Format("Directory \"{0}\" does not exist.", imageFolder));
             else
                 ImageFolder = null;
             _useImageType = type;