[core] Improve error message when style is not found or is incompatible

Attempting to use a Mapbox-hosted incompatible raster map/style with GL
would throw a simple 404, without any indication of the real issue.

Fix #1724, ref #1841

src/mbgl/map/map_context.cpp

@@ -118,6 +118,8 @@ void MapContext::setStyleURL(const std::string& url) {
 
         if (res.status == Response::Successful) {
             loadStyleJSON(res.data, base);
+        } else if (res.status == Response::NotFound && (styleURL.find("mapbox://") != std::string::npos)) {
+            Log::Error(Event::Setup, "style %s could not be found or is an incompatible legacy map or style", styleURL.c_str());
         } else {
             Log::Error(Event::Setup, "loading style failed: %s", res.message.c_str());
             data.loading = false;