Add 404 exception to remote service.

Former-commit-id: 3d7b0701acfce9be5d4725d4bea8b36f04de7bc1
Former-commit-id: 365df5cdb775013504aa3d7ced3167ea0226b731

src/ImageProcessor.Web/Services/RemoteImageService.cs

@@ -15,6 +15,7 @@ namespace ImageProcessor.Web.Services
     using System.IO;
     using System.Net;
     using System.Threading.Tasks;
+    using System.Web;
 
     using ImageProcessor.Web.Helpers;
 
@@ -159,6 +160,10 @@ namespace ImageProcessor.Web.Services
 
                             buffer = memoryStream.ToArray();
                         }
+                        else
+                        {
+                            throw new HttpException(404, "No image exists at " + uri);
+                        }
                     }
                 }
             }