`FlattenException` now unwraps errors

Contributed by Alexander M. Turek in #26028

Symfony wraps errors thrown by the application inside a FatalThrowableError. This makes the actual error class to not be displayed in the exception pages, where you see for example Symfony's FatalThrowableError instead of PHP's DivisionByZeroError when your code tries to divide by 0.

In Symfony 4.1, FlattenException now unwraps FatalThrowableError instances and logs the wrapped error. In consequence, the real error class is now always displayed in the exception page:

Introduced new exception classes

Contributed by Sullivan Senechal and Florent Mata in #25775 and #26475

In Symfony 4.1 we've introduced a new ProcessSignaledException class in the Process component to properly catch signaled process errors. Also, in the HttpFoundation component, we've introduced new detailed exception classes for file upload handling to replace the generic catch-all FileException:

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        use Symfony\Component\HttpFoundation\File\Exception\CannotWriteFileException;
use Symfony\Component\HttpFoundation\File\Exception\ExtensionFileException;
use Symfony\Component\HttpFoundation\File\Exception\FormSizeFileException;
use Symfony\Component\HttpFoundation\File\Exception\IniSizeFileException;
use Symfony\Component\HttpFoundation\File\Exception\NoFileException;
use Symfony\Component\HttpFoundation\File\Exception\NoTmpDirFileException;
use Symfony\Component\HttpFoundation\File\Exception\PartialFileException;
    

Moreover, now that PHP 7.1 supports multi catch exception handling, you can process several exceptions with the same catch() block:

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        try {
    // ...
} catch (FormSizeFileException | IniSizeFileException $e) {
    // ...
}
    

Improved the exception page design

Contributed by Javier Eguiluz in #26671

The exception pages have been improved in Symfony 4.1 to display less information about "vendor code". If some code belongs to the vendor/ folder, we compact its information to fit in a single line and we no longer display its arguments. The other code remains the same, which helps you focus more easily on your own application code: