Allow Postmark API exceptions to be thrown

[purplespider]

Integrated changes from ActiveCampaign@91afdf3

To enable, add the following to config.yml:

SilverStripe\Control\Email\SwiftMailer:
  swift_plugins:
    - Postmark\ThrowExceptionOnFailurePlugin

This means if the send failed due to the Postmark API not returning a success response, it will error, rather than pretending the email had sent sucessfully.