Adding unit tests, refactoring file access to a service, adding CatalogImageMissingException.

src/ApplicationCore/Exceptions/CatalogImageMissingException.cs

@@ -0,0 +1,13 @@
+using System;
+
+namespace ApplicationCore.Exceptions
+{
+    public class CatalogImageMissingException : Exception
+    {
+        public CatalogImageMissingException(string message, 
+            Exception innerException = null) 
+            : base(message, innerException: innerException)
+        {
+        }
+    }
+}