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.