Handle 204 No content responses correctly as an empty tile and avoid trying to decode the response

Nobody has claimed this yet.

Tileserver-GL started sending empty 204 responses if a tile is not available.
https://github.com/maptiler/tileserver-gl/blob/master/src/serve_data.js#L45
https://github.com/maptiler/tileserver-gl/pull/339

Mapbox-gl-js doesn't handle this very good. It creates an error message every time a tile is fetched and doesn't cache the server response (but the browser maybe does).

**mapbox-gl-js version**:
Since 1.7.0.
1.6.0 is not affected
**browser**:
Chrome, Firefox
### Steps to Trigger Behavior

1. Open the Demonstration
2. See Errors in Console
3. Scroll in and out. The 204 requests are repeated every time

### Link to Demonstration

https://jsbin.com/loyajulodu/edit?html,output

### Expected Behavior
* 204 response don't create a console error
* 204 response are cached inside mapbox-gl-js
### Actual Behavior
* 204 response create a console error
```
Error: Could not load image because of The source image could not be decoded.. Please make sure to use a supported image type such as PNG or JPEG. Note that SVGs are not supported.
at ajax.js:280
```
* 204 response are always passed to the Browser. The browser maybe caches it.

Start with the response handling near ajax.js:280 and reproduce the issue using the linked demonstration in Chrome or Firefox. Trace how a 204 response reaches image decoding, then verify that it is treated as an empty tile without a console error and is cached by mapbox-gl-js rather than requested repeatedly.

Written by the indexing model from the issue text.