FIX: Improve error handling for calculate_dominant_color! (#18503)

These errors tend to indicate that the upload is missing on the remote store. This is bad, but we don't want it to block the dominant-color calculation process. This commit catches errors when there is an HTTP error, and fixes the `base_store.rb` implementation when `FileHelper.download` returns nil.

lib/file_store/base_store.rb

@@ -116,6 +116,8 @@ module FileStore
             follow_redirect: true
           )
 
+          return nil if file.nil?
+
           cache_file(file, filename)
           file = get_from_cache(filename)
         end