Better handle for errors in transport

[Nightbr]

I think we should throw runtime error and only catch Mailjet API error. Because when we have a real issue (mailjet/mailjetBundle#10 with curl), we need to be alerted that there is an error...

Lets think about this.

https://github.com/mailjet/MailjetSwiftMailer/blob/master/SwiftMailer/MailjetTransport.php#L146 We can be more precise on the type of error we catch here. Also I think we should look at SwiftMailer recommendation and best practice on this.