[core] Don't use exceptions in MapObserver::onDidFailLoadingMap

Using different exception pointers to specify the loading failure makes
an awkward API. Most users rethrow the exception only to figure out what
type of error happened so it can be reported properly. So replace the
exception pointer with a enum an string description of the failure.

platform/node/src/node_map.cpp

@@ -3,8 +3,6 @@
 #include "node_feature.hpp"
 #include "node_conversion.hpp"
 
-#include <mbgl/map/projection_mode.hpp>
-#include <mbgl/util/exception.hpp>
 #include <mbgl/renderer/renderer.hpp>
 #include <mbgl/gl/headless_frontend.hpp>
 #include <mbgl/style/conversion/source.hpp>
@@ -26,6 +24,7 @@
 #include <mbgl/style/image.hpp>
 #include <mbgl/style/light.hpp>
 #include <mbgl/map/map_observer.hpp>
+#include <mbgl/util/exception.hpp>
 #include <mbgl/util/premultiply.hpp>
 
 #include <unistd.h>
@@ -54,8 +53,14 @@ static const char* releasedMessage() {
     return "Map resources have already been released";
 }
 
-void NodeMapObserver::onDidFailLoadingMap(std::exception_ptr error) {
-    std::rethrow_exception(error);
+void NodeMapObserver::onDidFailLoadingMap(mbgl::MapLoadError error, const std::string& description) {
+    switch (error) {
+        case mbgl::MapLoadError::StyleParseError:
+            Nan::ThrowError(NodeMap::ParseError(description.c_str()));
+            break;
+        default:
+            Nan::ThrowError(description.c_str());
+    }
 }
 
 void NodeMap::Init(v8::Local<v8::Object> target) {