The validation message was confusing by displaying only 1 level of
property of the service, even if the error was another level down.
Eg. if the 'files' property of 'extends' was the incorrect format,
it was displaying 'an invalid value for 'extends'', rather than
correctly retrieving 'files'.
Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>