Reading an invalid object should not throw any exceptions

[tindzk]

This currently prevents us from easily defining our own composite
readers, which is often necessary; see #31 and #68. Without the
exceptions, implicitly[Reader[_]].read calls can be chained using
orElse(), as done internally in uPickle.

sealed trait Base
case class Child1() extends Base
case class Child2() extends Base

import upickle._
implicit val BaseR: Reader[Base] = Reader[Base](
  implicitly[Reader[Child1]].read
    .orElse(implicitly[Reader[Child2]].read)
)

The exceptions are only useful for debugging purposes.

[lihaoyi]

Looks reasonable. In future it may be worth converting the PartialFunction[T, R] reads into a T => Either[Error, R] so we can preserve both the rich-error-data as well as the chainability