Make implicit API expectation implicit
Make implicit API expectation implicit

I was bitten by issue #42 when trying to implement a tilelive TileSource that used vector tiles, which by their nature are sparse. (My backend would faithfully emit error messages for missing tiles, but with a different wording which med tilelive crash out rather than just skip the tile.) Until the actual issue is resolved (which would mean rewriting all tilelive implementations, I guess) it should be made an explicit part or the API doc.
tomhughes/tilelive.js,topwood/tilelive,nyurik/tilelive.js,kyroskoh/tilelive,mapbox/tilelive.js,mapbox/tilelive