[8.x] Fix failover mailer when used with Mailgun & SES mailers

[gbuckingham89]

Description

#38344 introduced the failover mailer into the framework, with the idea being that if a mail fails to send via one mailer, alternative mailer(s) will be tried.

It seems that the first-party HTTP based mailers / transport included with the framework (Mailgun & SES) don't trigger an attempt to send using an alternative mailer if the API request fails - the exception from the underlying library isn't caught (a GuzzleException for Mailgun and AwsException for SES).

The issue seems to be that the send() method in the underlying \Swift_Transport_FailoverTransport class only catches Swift_TransportException exceptions, which trigger the failover logic. Other exceptions aren't handed.

Steps To Reproduce (with Mailgun):

1. Setup a failover mailer in config/mail.php:

'failover' => [
    'transport' => 'failover',
    'mailers' => [
        'mailgun',
        'log',
    ],
],

2. Ensure your attempt to send via Mailgun will fail (e.g. use incorrect credentials / block requests to the API domain on your local network).

3. Attempt to send a mail - you'll get aClientException exception, rather than the alternative failover log mailer being used.

Solution

The upstream swiftmailer/swiftamiler package is no longer maintained, so we can't suggest a PR to catch other exceptions in the failover logic. Plus this feels more like an issue with the Laravel transport classes not throwing the correct exceptions.

This PR simply catches any \GuzzleHttp\Exception\GuzzleException (Mailgun) / \Aws\Exception\AwsException (SES) exceptions and then throws a new Swift_TransportException instead (with the original passed in via $previous).

I haven't added any tests as there don't appear to be any for the MailgunTransport class, and wasn't sure of the best way of mocking a failure for SES - however I'm happy to add tests if anyone can assist!