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

src/Infrastructure/FileSystem/LocalFileImageService.cs

@@ -0,0 +1,22 @@
+using ApplicationCore.Interfaces;
+using Microsoft.AspNetCore.Hosting;
+using System.IO;
+
+namespace Infrastructure.FileSystem
+{
+    public class LocalFileImageService : IImageService
+    {
+        private readonly IHostingEnvironment _env;
+
+        public LocalFileImageService(IHostingEnvironment env)
+        {
+            _env = env;
+        }
+        public byte[] GetImageBytesById(int id)
+        {
+            var contentRoot = _env.ContentRootPath + "//Pics";
+            var path = Path.Combine(contentRoot, id + ".png");
+            return File.ReadAllBytes(path);
+        }
+    }
+}