Updating subject to be more descriptive

I've changed the email subject line to be the error message. When there are lots of errors being sent it is confusing to have them all grouped together in a mail client (like gmail) under the same conversation. This way, each error will have its own set of emails and can be searched/sorted/filtered in a mail client more easily.

Also I have fixed a bug where the exception class that was shown was always FlattenException. It now shows the original Exception class that threw the error.