Fix behavior of PhoneNumberToStringTransformer

The current implementation of ```PhoneNumberToStringTransformer::reverseTransform``` is incorrect: for each given string it should either return ```null``` or an instance of ```PhoneNumber```. If an error occurs while parsing the phone number, the transformer should throw a ```TransformationFailedException```. This will also automatically trigger the Form component to display a validation error.

The fact that it currently returns the input string when it can't parse it leads to strange behavior, especially when mapping to a property that is mapped as a doctrine phone_number type.