Add error message if relationship is missing for dot notation

[chrispenny]

If you created a cares for a relationship with dot notation, EG:

private static array $has_many = [
    'Links' => Link::class . '.MenuGroup',
];

private static array $cares = [
    'Links',
];

But there was no corresponding relationship on the other end (in this case, Link::class was missing a has_one), then an getRelationType() would return null, and Edge::__construct() would throw an unhelpful exception because $relationType must be string.