Add validation of langcode in MachineNameController

Problem/Motivation

In 10.2 we deprecated \Drupal\system\MachineNameController::transliterate
However it is possible to trigger a 500 exception by passing an array for the text or langcode query params.
We should catch that and return a 400 or similar rather than a 500.

This was originally reported to the security team but has been cleared to be a public issue.

Steps to reproduce

Proposed resolution

Add try/catch for \Symfony\Component\HttpFoundation\Exception\BadRequestException around these lines in \Drupal\system\MachineNameController::transliterate:

$text = $request->query->get('text');
    $langcode = $request->query->get('langcode');
    $replace_pattern = $request->query->get('replace_pattern');
    $replace_token = $request->query->get('replace_token');
    $replace = $request->query->get('replace');
    $lowercase = $request->query->get('lowercase');

This is a 10.x only issue as the controller has been removed in 11.x

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet