Put the error code from govdelivery in errors

Quite a few of the potential GovDelivery errors are documented as having
the same descriptive string, but different error codes.  Therefore,
let's put the error code into our exception messages, so we'll be able
to distinguish these.
Merge pull request #47 from alphagov/include-govdelivery-code-in-errors

Put the error code from govdelivery in errors