Add 404 exception to remote service.

Former-commit-id: 953c61e52764dbb100f01b19399515e230fe295d
Former-commit-id: b867fdc358c972522e74def500515f704bf5f5ca

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);
+                        }
                     }
                 }
             }