Add response body message to the exception

We should add the response body message to the exception in order to understand better which is the Mailgun error, in my specific case i've spent a lot time digging around the code/configs till the var_dump() of the Exception pointed me out the needed messaage: "Check your inputs! 'from' parameter is not a valid address. please check documentation." which was shallowed by "Exception occurred while trying to send test email from Zambon to HIDDEN. 400: The parameters passed to the API were invalid".