adding exception logging

[levijackson]

This taps into the responseReceived event and it logs code + message (401: Cannot send message without a recipient). #34

I'm using this outside of Symfony so my manual test looks like this:

$domain = 'test.com';
// Use incorrect key to verify the error is logged
$apiKey = 'API_KEY';

$eventDispatch = new \Swift_Events_SimpleEventDispatcher();
$mailgun = \Mailgun\Mailgun::create($apiKey);
$transport = new \cspoo\Swiftmailer\MailgunBundle\Service\MailgunTransport($eventDispatch, $mailgun, $domain);
$mailer = Swift_Mailer::newInstance($transport);

$swiftReporter = new Swift_Plugins_Reporters_HitReporter();
$mailer->registerPlugin(new Swift_Plugins_ReporterPlugin($swiftReporter));

$swiftLogger = new Swift_Plugins_Loggers_ArrayLogger();
$mailer->registerPlugin(new Swift_Plugins_LoggerPlugin($swiftLogger));

$message = Swift_Message::newInstance();
$message->setTo('foo@foo.com')
    ->setFrom('bar@bar.com')
    ->setSubject('Foo bar')
    ->setBody('test message');

$result = $mailer->send($message);

pr($swiftLogger->dump());

[codecov-io]

Codecov Report

Merging #56 into master will decrease coverage by 0.3%.
The diff coverage is 80%.

@@             Coverage Diff              @@
##             master      #56      +/-   ##
============================================
- Coverage     86.66%   86.36%   -0.31%     
- Complexity       26       28       +2     
============================================
  Files             3        3              
  Lines           105      110       +5     
============================================
+ Hits             91       95       +4     
- Misses           14       15       +1

Impacted Files	Coverage Δ	Complexity Δ
Service/MailgunTransport.php	80.82% <80%> (-0.07%)	23 <0> (+2)

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