[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.

2 files changed, 4 insertions, 3 deletions

diff --git a/test/sprite/sprite_parser.cpp b/test/sprite/sprite_parser.cpp
index dfa3a1055b..0eb298b23e 100644
--- a/test/sprite/sprite_parser.cpp
+++ b/test/sprite/sprite_parser.cpp
@@ -5,6 +5,7 @@
 #include <mbgl/sprite/sprite_image.hpp>
 #include <mbgl/util/image.hpp>
 #include <mbgl/util/io.hpp>
+#include <mbgl/util/string.hpp>
 
 #include <algorithm>
 
@@ -211,9 +212,9 @@ TEST(Sprite, SpriteParsingInvalidJSON) {
     const auto image_1x = util::read_file("test/fixtures/annotations/emerald.png");
     const auto json_1x = R"JSON({ "image": " })JSON";
 
-    const auto error = parseSprite(image_1x, json_1x).get<std::string>();
+    const auto error = parseSprite(image_1x, json_1x).get<std::exception_ptr>();
 
-    EXPECT_EQ(error,
+    EXPECT_EQ(util::toString(error),
               std::string("Failed to parse JSON: Missing a closing quotation mark in string. at offset 13"));
 }
 
diff --git a/test/sprite/sprite_store.cpp b/test/sprite/sprite_store.cpp
index 574af51b81..6209bde2a3 100644
--- a/test/sprite/sprite_store.cpp
+++ b/test/sprite/sprite_store.cpp
@@ -176,7 +176,7 @@ public:
         callback_(spriteStore_.get(), nullptr);
     }
 
-    void onSpriteLoadingFailed(std::exception_ptr error) override {
+    void onSpriteError(std::exception_ptr error) override {
         callback_(spriteStore_.get(), error);
     }