Display the value of layer.id on validation

[tristen]

Right now - If I write a bad value for a filter - the errors returned look like this:

Displaying the array index layer[125] is less helpful if I'm writing a style in a text editor or my style is compiled from a bunch of layer scripts into one stylesheet.

It would be very helpful if the name of the layer (in this case a layer I've named transit-station-label was displayed. Then I would have string to search / I could quickly identify the mistake I've made in authoring.

[asheemmamoowala]

The specific instance that can be fixed for this error is:

mapbox-gl-js/src/style-spec/validate/validate_object.js

Lines 38 to 46 in a36fc99

	errors = errors.concat(validateElement({
	key: (key ? `${key}.` : key) + objectKey,
	value: object[objectKey],
	valueSpec: elementSpec,
	style,
	styleSpec,
	object,
	objectKey
	}, object));

The validation logic is pretty generic, and requires special casing the error message generation to use an id field when available. Ideally, the use of the layer.id can be applied to all layer fields, and not just the filter