…wMessage without an Envelope (fabpot)

This PR was merged into the 4.4 branch.

Discussion
----------

[Mailer] Improve an exception when trying to send a RawMessage without an Envelope

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | #33256
| License       | MIT
| Doc PR        | n/a

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 - Bug fixes must be submitted against the lowest maintained branch where they apply
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 - Features and deprecations must be submitted against branch 4.4.
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Commits
-------

c6d56de [Mailer] Improve an exception when trying to send a RawMessage without an Envelope

This PR was merged into the 4.4 branch.

Discussion
----------

[Mailer] Added messenger to dev dependencies

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

PR #33454 introduced a new test case that mocks the `MessageBusInterface` from the messenger component. This makes the tests suite fail on travis because PHPUnit refuses to create a mock for an interface that does not exist.

```
There was 1 failure:

1) Symfony\Component\Mailer\Tests\MailerTest::testSendingRawMessages

Failed asserting that exception of type "PHPUnit\Framework\MockObject\RuntimeException" matches expected exception "Symfony\Component\Mailer\Exception\LogicException". Message was: "Cannot stub or mock class or interface "Symfony\Component\Messenger\MessageBusInterface" which does not exist" at

/home/travis/build/symfony/symfony/src/Symfony/Component/Mailer/Tests/MailerTest.php:28
```

This PR fixes the test by adding the Messenger component to Mailer's dev dependencies.

Commits
-------

248f082 [Mailer] Added messenger to dev dependencies.