Relax private constraint for exception classes.

[zainab-ali]

#192 made some classes private, and hasn't been released. I tested it out in a few more codebases, and have found some new use cases for having public exception types.

Pattern matching

While users shouldn't construct exceptions explicitly, they may want to pattern match on them:

match {
  case _: IgnoredException => ...
  case _: ExpectationFailed => ...
  case other => ...
}

Control flow with Either

Users might also want to explicitly construct ExpectationFailed for control flow. For example, the following code can't be written using our public API:

sealed trait Pet
case class Cat(name: String) extends Pet
case class Dog(name: String) extends Pet

def dogOrFailure(pet: Pet): Either[ExpectationFailed, Dog] = pet match {
  case dog: Dog => Right(dog)
  case _ => Left(new ExpectationFailed("not a dog", NonEmptyList.of(implicitly[SourceLocation]))
}

I don't think we should expose the ExpectationFailed constructor in the long term, but can do so in the short term and replace it with a helper function in future.