Timezone issue in BaseDateTimeTransformer

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9403
| License       | MIT
| Doc PR        | 

In BaseDateTimeTransformer constructor we need to check if timezone is valid.

DateTimeZone object creation could rise an exception. And if this exception is catched and rethrowed as TransformationFailed exception we have an issue: user will see "Invalid value" error in form, but issue is actually with php configuration or form type configuration.

See example below:

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/Core/DataTransformer/DateTimeToArrayTransformer.php#L176