[Form] Improved exception message if the data class is not found

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12359
| License       | MIT
| Doc PR        | -
It seems like PR is based on 2.3. For 2.5 and 2.6 it would be a valid message.
https://github.com/symfony/symfony/blob/05815adb227eb3adf891a60ec1eb27d8d64f51d6/src/Symfony/Component/Form/FormConfigBuilder.php#L195