setData() with invalid GeoJSON fails in two different ways, or doesn't fail

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**mapbox-gl-js version**: 1.6.1

**browser**:

### Steps to Trigger Behavior

```js
map.getSource('mygeojson').setData({ not: 'geojson'})

Object { message: "Input data is not a valid GeoJSON object.", stack: "" }
mapbox-gl.js:29

map.getSource('mygeojson').setData({ type: 'Point'})

Object { message: "e is undefined", stack: "" }
mapbox-gl.js:29

map.getSource('mygeojson').setData({ type: 'Point', coordinates: { lng: 30, lat: 20 }})
/* no error thrown, data is actually saved but doesn't display */
```

So, yeah. This is quite odd. Depending on exactly what is wrong with the GeoJSON that you try to set either:

1. You get a nice exception with a message.
2. You get an exception with no message.
3. You get no exception at all, and the data is actually saved, but simply fails to display anything.

The third case is particularly nasty. Took me quite a while to realise what I was doing wrong.

### Expected behaviour

The first exception thrown in all three cases.

Start at the GeoJSON source setData() entry point and reproduce the three invalid payloads shown in the issue. Trace their validation paths and add or update the relevant tests so every malformed input raises a consistent, useful exception rather than being silently accepted.

Written by the indexing model from the issue text.