Represent validation errors as data

`refine` currently returns an `Either[String, A]`. `String` is an awful type for errors and should probably be replaced by a dedicated error type like `RefinementError`.

``` scala
case class RefinementError(msg: String)
```

Start at the refine API and trace its callers to determine how the current Either[String, A] error value is used. Replace the string error representation with the proposed dedicated error type, and confirm that affected callers and behavior remain consistent.

Written by the indexing model from the issue text.