Writing array-valued columns to "ascii" format crashes in FastWriter

Per this StackOverflow question: http://stackoverflow.com/questions/29826828/how-do-i-write-a-3d-astropy-table-to-a-file

I think this shouldn't just work, since there's no straightforward way to write this kind of data to an arbitrary ASCII format. But if nothing else the use case should be prevented early on, rather than crashing in an unhelpful way.

In other words: We need a better error message.