message: reactor.network: finish the migration from Exception to Error
diff: @@ -48,10 +48,10 @@ namespace elle
  /// Exception thrown when DNS invited us to retry.
  class TryAgain
  - : public Exception
  + : public Error
  {
  public:
  - using Super = Exception;
  + using Super = Error;
  TryAgain()
  : Super{"Host not found (non-authoritative), try again later"}
  {}
  