Changing map style causes plugin to throw errors

Nobody has claimed this yet.

This bug was initially filed as https://github.com/mapbox/mapbox-gl-js/issues/7019 but I'm moving it here because the bug is in the plugin.

When you change a map's style, the route feature is removed causing this line to throw the error `Error: The layer 'directions-route-line-alt' does not exist in the map's style and cannot be queried for features.`

https://github.com/mapbox/mapbox-gl-directions/blob/31c7911a616a0e2e306742b30d87e33cdfd56c8f/src/directions.js#L304-L312

### Steps to Trigger Behavior

1. Add directions route to map
2. Change base map style
3. Move mouse around the map

### Link to Demonstration
https://plnkr.co/edit/M8mj1WTH1KjDa1dhiUKw?p=preview

### Expected Behavior
The plugin should be able to handle this situation so that no errors are thrown.

### Actual Behavior
Errors are thrown.

cc @mikeomeara1

Start in src/directions.js at lines 304-312 and reproduce the reported sequence: add a directions route, change the base map style, then move the mouse. Verify the plugin's behavior when the route feature or queried layer is absent. Done means changing the style and moving around the map no longer throws errors.

Written by the indexing model from the issue text.