Consider using "checked exceptions" for client APIs

Not like Java. Just had to get that out of the way.

This blog post and the library he's talking about demonstrates a pretty cool way of keeping exceptions in the types: https://functor.tokyo/blog/2017-05-10-servant-checked-exceptions#.WR5lMEsqJ-4.reddit

I.e. instead of putting all possible exception cases in `RelationalError`, you could pull them out logically and use his API as an example of how to do make a cleaner client API. Possibly.