Improve v1 > v2 error messages

[martinjbaker]

I just spent quite a few hours going around in circles because the error message is not helpful.

If you send a v1 "send email" request that has the data wrapped in "transmissionBody" to the v2+ module then the error given is

errors:
[ { message: 'required field is missing',
description: 'content object or template_id required',
code: '1400' } ],
statusCode: 422

To stop users screaming at the computer screen when they can see that "content" IS actually set, it would be more helpful if the module first identified that "transmissionBody" was at the top level of the data and posts an error that it is no longer required for v2.