Make "Map#load" fire, even if "Not Found" errors were encountered

[lucaswoj]

ref #1985
ref #1800

If a vector tile request gets a 404 response, the Map#load event doesn't fire.
#1800 is about a feature request that is different than this bug but the two have been conflated.

cc @jfirebaugh @ansis @geografa @laurenancona

[ansis]

looks good to me

[superphly]

So by setting tile.errored = true;, that allows the map#load to fire?

[lucaswoj]

@superphly Correct. To understand why, take a look at the TilePyramid#loaded method https://github.com/mapbox/mapbox-gl-js/blob/master/js/source/tile_pyramid.js#L49

[laurenancona]

\o/

Thanks, @lucaswoj