return an error if map.renderStill returns a std::exception_ptr

store exception_ptr on NodeMap

add test for map.render returning an error

src/node_map.hpp

@@ -50,6 +50,7 @@ private:
     NodeFileSource &fs;
     mbgl::Map map;
 
+    std::exception_ptr error;
     std::unique_ptr<const mbgl::StillImage> image;
     std::unique_ptr<NanCallback> callback;