exceptions are not raised for 5XX errors from SendGrid

[hrtshu]

exceptions are not raised for 5XX errors from SendGrid even with raise_delivery_errors option

sendgrid-actionmailer/lib/sendgrid_actionmailer.rb

Lines 295 to 298 in 135d244

	if result.status_code && result.status_code.start_with?('4')
	full_message = "Sendgrid delivery failed with #{result.status_code}: #{result.body}"
	settings[:raise_delivery_errors] ? raise(SendgridDeliveryError, full_message) : warn(full_message)
	end

Why only for 4XX erros?

If exceptions are not raised for 5XX errors, it behaves as if it has succeeded.
So we can't notice SendGrid incidents like this even if they occur.

And ActionJob retry or AWS SQS retry or something won't be triggered.

Here's the simple solution: #119