Terrain (RGB-encoded DEM) datasource returns 404 for empty tiles

[sukhanovr]

mapbox-gl-js version: 2.0.0

browser: Chrome, Firefox

Steps to Trigger Behavior

1. Use style with Terrain (RGB-encoded DEM) source and layer
2. Navigate to any sea area

Link to Demonstration

https://jsfiddle.net/m6k791hj/6/
Coordinates are set to [0, 0], middle of the ocean, and there is of course no tiles for that region so tileset returns multiple 404.

Expected Behavior

As claimed in the issue tilesets should return 204 (No Content) for such cases

Actual Behavior

404 (Not Found) is returned

[JuanIrache]

If this happens the "idle" event does not fire, so if our program depends on it it gets stuck. Any workarounds? Any way to make these failed requests fire the "error" event?