[core] Rationalize error handling for resource loading

* Standardize on std::exception_ptr as the error representation
  (fixes #2854).
* Don't format textual strings at the error source; pass on the
  constituent data via observer method parameters instead.
* Use the null object pattern to simplify observer notification code.
* Further refactoring for ResourceLoading tests.

1 files changed, 3 insertions, 5 deletions

diff --git a/src/mbgl/map/map_context.hpp b/src/mbgl/map/map_context.hpp
index 24459f270a..012da785c9 100644
--- a/src/mbgl/map/map_context.hpp
+++ b/src/mbgl/map/map_context.hpp
@@ -65,14 +65,12 @@ public:
     void onLowMemory();
 
     void cleanup();
-
-    // Style::Observer implementation.
-    void onTileDataChanged() override;
-    void onResourceLoadingFailed(std::exception_ptr error) override;
-
     void dumpDebugLogs() const;
 
 private:
+    void onResourceLoaded() override;
+    void onResourceError(std::exception_ptr) override;
+
     // Update the state indicated by the accumulated Update flags, then render.
     void update();