`failFast` only reports the first failure

[zainab-ali]

failFast only reports the first failure in a group of expectations. It is defined as follows:

  def failFast[F[_]: Sync]: F[Unit] =
    this.run match {
      case Invalid(e) => Sync[F].raiseError(e.head)
      case Valid(_)   => Sync[F].unit
    }

To support multiple failures, we could define a new datatype for raising composite exceptions. For example:

case class ExpectationsFailed(failures: NonEmptyList[ExpectationFailed]) extends Throwable

This could be matched against when constructing a Result.