Update mailer.rst

[ThomasLandauer]

Follow-up of symfony/symfony#37313 (comment)

Explaining how to "verify" successful delivery by catching the TransportException

[ThomasLandauer]

Don't know what went wrong at "CI / Build"

[javiereguiluz]

Thomas, thanks a lot for this contribution. This was an important missing thing in the docs. While merging I did some tweaks though.

First, I renamed the section from Catching Transport Exceptions to a more generic Handling Sending Failures, which can be expanded later with more problems and/or solutions.

Then, I moved the caution message at the top of the section and turned it into normal text ... because I think it's important to explain what Symfony considers successful and failed email sending.