Surface attachment derivative errors

[zdavis]

• Promote the attachment right away
• Continue to process derivatives in the background
• Ensure that client continues to show processing placeholder until derivatives exist
• Use Shrine add_metadata plugin to store derivative errors
• Expose derivative errors in the serializer
• Render derivative errors underneath attachment fields

Derivatives job could look like this:

module Attachments
  class CreateDerivativesJob < ApplicationJob
    queue_as :processing
    discard_on ActiveJob::DeserializationError
    def perform(record, attachment_name)
      uploaded_file = record.public_send(:"#{attachment_name}_attacher")
      uploaded_file.create_derivatives # i think this is the method, may need something slightly more since we're no longer promoting here
      uploaded_file.add_metadata has_derivatives: true, derivatives_failed_at: nil, derivative_failure_reason: nil, derivative_failure_exception_klass: nil
      uploaded_file.write
      record.save!
    rescue SomeException => e
      uploaded_file.add_metadata has_derivatives: false, derivatives_failed_at: Time.current, derivative_failure_reason: e.message, derivative_failure_exception_klass: e.class.name
      uploaded_file.write
      record.save!
    end
  end
end