Add stoplight for object storage failures, return HTTP 503 (#13043)

config/initializers/paperclip.rb

@@ -113,3 +113,14 @@ else
 end
 
 Paperclip.options[:content_type_mappings] = { csv: Import::FILE_TYPES }
+
+# In some places in the code, we rescue this exception, but we don't always
+# load the S3 library, so it may be an undefined constant:
+
+unless defined?(Seahorse)
+  module Seahorse
+    module Client
+      class NetworkingError < StandardError; end
+    end
+  end
+end