…cheme (Koc)This PR was merged into the 4.4 branch.Discussion----------[Mailer][DX] Improve exception message for unsupported scheme| Q             | A| ------------- | ---| Branch?       | 4.4| Bug fix?      | no| New feature?  | yes| BC breaks?    | no| Deprecations? | no| Tests pass?   | waiting for Travis| Fixed tickets | -| License       | MIT| Doc PR        | -This PR improves exception message for unsupported schemes by providing list of available. Throw something like: ` 'The "foo" scheme is not supported for mailer "mailgun". Supported schemes are: "api", "http", "smtp".'`Commits-------8c24a53 [Mailer][DX] Improve exception message for unsupported scheme

…es (Koc)This PR was squashed before being merged into the 4.4 branch (closes#32609).Discussion----------[Mailer][DX][RFC] Rename mailer bridge transport classes| Q             | A| ------------- | ---| Branch?       | 4.4| Bug fix?      | yno| New feature?  | no| BC breaks?    | yes| Deprecations? | no| Tests pass?   | yes| Fixed tickets | -| License       | MIT| Doc PR        | -During working on#31946 I realized how painful to work with multiple classes which has same name. [Nice article](https://www.tomasvotruba.cz/blog/2019/05/02/alias-as-a-code-smell/) by@TomasVotruba with explanation of problems  with such approach.~Built on top of#32608 , so only [2nd commit](bbf7e99) is actual.~Also I've changed namespaces to make bridge structure much simpler and be linear. All classes located on same level now. See how [bridge](https://github.com/symfony/symfony/tree/bbf7e99e89c70fab372929827ae509b41280ce40/src/Symfony/Component/Mailer/Bridge/Amazon) looks like now.Now in RFC state to get approve for such king of changes and update all other bridges.Commits-------eda4f01 [Mailer][DX][RFC] Rename mailer bridge transport classes