[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.

src/mbgl/map/map_impl.cpp

@@ -2,6 +2,7 @@
 #include <mbgl/map/map_impl.hpp>
 #include <mbgl/renderer/update_parameters.hpp>
 #include <mbgl/style/style_impl.hpp>
+#include <mbgl/util/exception.hpp>
 
 namespace mbgl {
 
@@ -97,7 +98,26 @@ void Map::Impl::onStyleLoaded() {
 }
 
 void Map::Impl::onStyleError(std::exception_ptr error) {
-    observer.onDidFailLoadingMap(error);
+    MapLoadError type;
+    std::string description;
+
+    try {
+        std::rethrow_exception(error);
+    } catch (const mbgl::util::StyleParseException& e) {
+        type = MapLoadError::StyleParseError;
+        description = e.what();
+    } catch (const mbgl::util::StyleLoadException& e) {
+        type = MapLoadError::StyleLoadError;
+        description = e.what();
+    } catch (const mbgl::util::NotFoundException& e) {
+        type = MapLoadError::NotFoundError;
+        description = e.what();
+    } catch (const std::exception& e) {
+        type = MapLoadError::UnknownError;
+        description = e.what();
+    }
+
+    observer.onDidFailLoadingMap(type, description);
 }
 
 #pragma mark - Map::Impl RendererObserver