Now it works more like StubStyleObserver: you can assign std::functions to specific slots based on resource type.
Rewrite resource loading tests in that style, making them less like integration tests of Style and more like unit tests of Source, GlyphStore, and SpriteStore.
* 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.
