[core] keep tiles renderable even if a subsequent error occurs

Since 9a9408e811, we marked tiles as non-renderable if an error occured. This lead to situations where a tile was loaded + parsed successfully, then a revalidation attempt occured (e.g. because the resource was stale) which failed. In this case, we used to mark the tile as non-renderable although we could've used the perfectly parsed (stale) resource.

src/mbgl/tile/raster_tile.cpp

@@ -29,7 +29,6 @@ void RasterTile::cancel() {
 
 void RasterTile::setError(std::exception_ptr err) {
     loaded = true;
-    renderable = false;
     observer->onTileError(*this, err);
 }
 
@@ -49,9 +48,7 @@ void RasterTile::onParsed(std::unique_ptr<RasterBucket> result) {
 }
 
 void RasterTile::onError(std::exception_ptr err) {
-    bucket.reset();
     loaded = true;
-    renderable = false;
     observer->onTileError(*this, err);
 }