Return server response as a text instead of statusText

Nobody has claimed this yet.

## Motivation
Right now, when a vector tile fails to load, Mapbox is returning an error with the statusText which in most cases is empty.
statusText is ambiguous with http/2

Here is a link to the piece of code I would like to change
https://github.com/mapbox/mapbox-gl-js/blob/94e4714c0ab2896304d72e6322b30fe0bad392a5/src/util/ajax.js#L230
We can change it to
```js
callback(new AJAXError(xhr.responseText || xhr.statusText, xhr.status, requestParameters.url));
```

## Design Alternatives

## Design

### Mock-Up

### Concepts

### Implementation

Start in src/util/ajax.js around line 230, the entry point linked in the issue, and inspect how failed vector-tile requests construct AJAXError. Done means the error message uses the server response text when available and retains the status-text fallback.

Written by the indexing model from the issue text.