Add Schema.Fail

[jdegoes]

Schema.Fail will be the schema that always fails. It represents the absence of schema information for a given type.

Something like:

object Schema {
  final case class Fail[A](message: String) extends Schema[A]
  def fail[A](message: String): Schema[A] = SchemaFail(message)
}

[thinkharderdev]

Can I take this one?