[Config] Allow extra hint in exception message

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #1666
| License       | MIT
| Doc PR        | symfony/symfony-docs#4047
not a fan of assignments in conditions but then again I guess we do this in tons of other places