Add useInterimTilesOnError property to ol.layer.Tile
Activate useInterimTilesOnError in ol.renderer.canvas.TileLayer
Activate useInterimTilesOnError in ol.renderer.webgl.TileLayer
Activate useInterimTilesOnError in ol.renderer.dom.TileLayer
Merge pull request #1757 from twpayne/interim-tiles-on-error

Add option to use interim tiles on tile errors