uploads: address "Failed to replace upload_media_assets..." error

Sometimes uploads fail with this error:

    Failed to replace upload_media_assets because one or more of the new
    records could not be saved.

Change it so that media assets are saved individually, so that if saving
any of them fails we get a better error message.

app/models/upload.rb

@@ -114,7 +114,7 @@ class Upload < ApplicationRecord
 
     if files.present?
       upload_media_assets = files.map do |_index, file|
-        UploadMediaAsset.new(file: file.tempfile, source_url: "file://#{file.original_filename}")
+        UploadMediaAsset.new(upload: self, file: file.tempfile, source_url: "file://#{file.original_filename}")
       end
     elsif source.present?
       page_url = source_extractor.page_url
@@ -125,13 +125,16 @@ class Upload < ApplicationRecord
       end
 
       upload_media_assets = image_urls.map do |image_url|
-        UploadMediaAsset.new(source_url: image_url, page_url: page_url, media_asset: nil)
+        UploadMediaAsset.new(upload: self, source_url: image_url, page_url: page_url, media_asset: nil)
       end
     else
       raise Error, "No file or source given" # Should never happen
     end
 
-    update!(upload_media_assets: upload_media_assets, media_asset_count: upload_media_assets.size)
+    transaction do
+      update!(media_asset_count: upload_media_assets.size)
+      upload_media_assets.each(&:save!)
+    end
   rescue Exception => e
     update!(status: "error", error: e.message)
   end