Handling exception in openlayers 3 when loading GeoJSON, how?

Question

Geolocation

How to catch an Exception in Openlayers 3 when loading GeoJSON from the server... for example, if the server suddenly became unavailable or the geojson was formed clumsily, or the database fell off .... in short, so that you can spit the error. Nowhere did I find an answer to the question.

var vs = new ol.source.Vector({
    url:'geojson.php',
    format: new ol.format.GeoJSON() 
});
var vl = new ol.layer.Vector({
    source:vs,
    style:styleFunction
});
map.addLayer(vl);