Allow CiviCRM Email Processor to handle Multipart Report messages

We have been using sendgrid as an email delivery service. When they have been sending back bounces to our IMAP folder they have been in the form of a multipart Report.

This has meant all we have been seeing on the bounce report in CiviCRM has been

"This is an automatically generated message from SendGrid. I'm sorry to have to tell you that your message was not able to be delivered to one of its intended recipients. If you require assistance with this, please contact SendGrid support."

We need to handle multipart Report messages better than just grabbing the first few lines of the text