[Twig] fixed TwigEngine::exists() method when a template contains a syntax error (closes #8546)

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8546
| License       | MIT
| Doc PR        | n/a