Add option to use interim tiles on tile errors

This PR adds a `useInterimTileOnError` option to `ol.layer.Tile` (default `true`) that causes the render to use an interim (lower resolution) tile if the current tile has an error (e.g. it doesn't exist). This provides a better visual experience when tiles are not present on the server and ensures that pixels are always drawn.

@asaunier, this is what you requested for RTD.