Fixed: Don't try to create metadata images if source files doesn't exist

(cherry picked from commit 9a1022386a031c928fc0495d6ab990ebce605ec1)

Closes #4132

src/NzbDrone.Core/Extras/Metadata/MetadataService.cs

@@ -470,6 +470,7 @@ namespace NzbDrone.Core.Extras.Metadata
         private void DownloadImage(Artist artist, ImageFileResult image)
         {
             var fullPath = Path.Combine(artist.Path, image.RelativePath);
+            var downloaded = true;
 
             try
             {
@@ -477,13 +478,20 @@ namespace NzbDrone.Core.Extras.Metadata
                 {
                     _httpClient.DownloadFile(image.Url, fullPath);
                 }
-                else
+                else if (_diskProvider.FileExists(image.Url))
                 {
                     _diskProvider.CopyFile(image.Url, fullPath);
                 }
+                else
+                {
+                    downloaded = false;
+                }
 
+                if (downloaded)
+                {
                     _mediaFileAttributeService.SetFilePermissions(fullPath);
                 }
+            }
             catch (WebException ex)
             {
                 _logger.Warn(ex, "Couldn't download image {0} for {1}. {2}", image.Url, artist, ex.Message);