Render error tiles as empty

Previously, if a tile load resulted in an error the pixels in the rendered map would have an indeterminate state. [Discussion here](https://groups.google.com/d/msg/ol3-dev/dZCj_kwaZS8/_GgGujzqBUIJ).

With this PR, tiles that error are drawn as empty tiles and properly cleared.

I'm not sure whether the second commit, c576b2a, is useful.