Make loadFeaturesFromUrl accept an error callback
Make createTileCoordTransform return identity func

This makes ol.tilegrid.TileGrid#createTileCoordTransform return the identity function. This makes it possible to use ol.source.TileVector with an ol.tilegrid.TileGrid (as opposed to an ol.tilegrid.XYZ).
Merge pull request #2971 from elemoine/tilevector

Two ol.source.TileVector fixes