MailjetApiHandler debug mode messages don't output data properly

Problem/Motivation

When debugging sent mails, the @data parameter is a nested array which doesn't get output properly.
The logger expects variables to be a string, but $response->getData() is a nested array instead.

Also, the protocol will be spammed with warnings whenever the logged message is tried to be rendered.

Steps to reproduce

Install & setup mailjet with a demo account
Activate debugging mode
Write a mail_alter hook and purposefully malform the $message. (In my case, I added a NULL variable like so: $message['param']['Variables']['dummy'] = NULL)
Set up the mailmanager to use mailjet
Send a test mail via mailmanager

Proposed resolution

print_r the $response->getData()

Remaining tasks

User interface changes

API changes

Data model changes