Add error handling mechanism to higher-level processing methods.

[satabin]

Idea from #13 (comment)

Currently the process methods fail and nack upon any exception raised in the processing.
The idea is to give users a way to make decisions upon failures. This can be used to implement exponential backoff, dead-lettering of malformed messages, etc...

The basic abstraction could look like:

// this is a SAM, so any function Throwable => F[Decision] will do
// the `F` allows for logging or any other effectful action suited for the handler
trait ErrorHandler[F[_]] {
  def handle(msg: RawMessage, t: Throwable): F[Decision]
}

sealed trait Decision
object Decision {
  case object Drop extends Decision
  case class Fail(t: Throwable, ack: Boolean) extends Decision
  case object DeadLetter extends Decision
  case class Reenqueue(delay: Option[FiniteDuration]) extends Decision
}

We can provide a sensible default handler (nacking on any error except for deserialization errors for instance).