[OptionsResolver] fix count() and exceptions

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Please see commits.
In my opinion the definition is just too strict on the php site. In reality it's also used for other cases where the name makes sense. Even you did that: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Validator/Context/ExecutionContext.php#L190