Change symbol in validation error for Either from every? to either?. Add
the explanation of schemas to validation error for Either to improve feedback.
After discussing the semantics of the walker with Jason Wolfe, `either?` has been replaced with `some` in the walker for `Either`, and the verbose expansion of the schemas has been removed.
Merge pull request #71 from tgk/improve-either

Make the Either schema validation error easier to understand.