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.
Updating glyphs is still unsupported, and there's no good use case for doing so. When we're using a stale glyph PBF, and the fresh answer contains changed to that glyph, we will continue to use the old glyph.
* 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.
This will make the code a lot more clear and it will
also move how parsing is initiated to the GlyphPBF
class, to be initiated after the request, like we do
for other resources.
