[Mailer] Include all transports' debug messages in RoundRobin transport exception #48126
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Description
The RoundRobin Mail transport does not include inner transports' debug messages in case of failure, which makes it very difficult to reason about the failures.
This PR attempts to add that debug information to the thrown TransportException.
In the process of fixing this issue, I discovered that one of the existing tests is incorrect. The last assertion is never executed and the test passes regardless of the assertion state. The test was also fixed.
Note that although this issue exists in https://github.com/symfony/symfony/tree/5.0, I wasn't able to apply the same patch to it since the behavior of the transport (specifically the cursor property) was different. I can open a new PR to fix the same issue for that branch as well.