MySQL exit handler ignored

An exit handler is being ignored by my stored procedure. I execute the following line:

CALL updateTemplate('MyObject', 'NewTemplate');

And receive the error:

Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (db.objects, CONSTRAINT Object: Template Foreign Key FOREIGN KEY (TemplateId) REFERENCES templates (TemplateId) ON UPDATE CASCADE)

Indeed, a constraint does fail, because 'NewTemplate' does not exist in the template table. But why do I not receive the custom error message in the exit handler of the stored procedure below?

CREATE DEFINER=`root`@`localhost` PROCEDURE `updateTemplate`(in inObjectId varchar(45), in inTemplateId varchar(45))
BEGIN

    declare exit handler for 1452
    begin
        signal sqlstate '45000' set MESSAGE_TEXT = 'The template identifier is invalid.';
    end;

    UPDATE objects SET TemplateId=inTemplateId WHERE ObjectId=inObjectId;

END

For info, I'm running MySQL 5.6.20.