[core] Emit MapChangeDidFailLoadingMap when the style cannot be loaded or parsed

Currently this signal is never emitted, which can cause the Still mode
to starve in case of an invalid style or failed request.

src/mbgl/map/map.cpp

@@ -38,6 +38,7 @@ public:
     Impl(View&, FileSource&, MapMode, GLContextMode, ConstrainMode, ViewportMode);
 
     void onNeedsRepaint() override;
+    void onStyleError() override;
     void onResourceError(std::exception_ptr) override;
 
     void update();
@@ -316,6 +317,8 @@ void Map::setStyleURL(const std::string& url) {
             } else {
                 Log::Error(Event::Setup, "loading style failed: %s", res.error->message.c_str());
             }
+            impl->onStyleError();
+            impl->onResourceError(std::make_exception_ptr(std::runtime_error(res.error->message)));
         } else if (res.notModified || res.noContent) {
             return;
         } else {
@@ -343,8 +346,8 @@ void Map::setStyleJSON(const std::string& json) {
 }
 
 void Map::Impl::loadStyleJSON(const std::string& json) {
-    style->setJSON(json);
     style->setObserver(this);
+    style->setJSON(json);
     styleJSON = json;
 
     // force style cascade, causing all pending transitions to complete.
@@ -908,6 +911,10 @@ void Map::Impl::onNeedsRepaint() {
     asyncUpdate.send();
 }
 
+void Map::Impl::onStyleError() {
+    view.notifyMapChange(MapChangeDidFailLoadingMap);
+}
+
 void Map::Impl::onResourceError(std::exception_ptr error) {
     if (mode == MapMode::Still && callback) {
         callback(error, {});