[2.6][TwigBundle] use "Debug\Exception\FlattenException" instead of "HttpKernel\Exception\FlattenException"

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12965
| License       | MIT