message: Add `sec::none` (not-an-error) for convenience
diff: @@ -31,6 +31,8 @@ namespace caf {
  /// SEC stands for "System Error Code". This enum contains
  /// error codes used internally by CAF.
  enum class sec : uint8_t {
  + /// No error.
  + none = 0,
  /// Indicates that an actor dropped an unexpected message.
  unexpected_message = 1,
  /// Indicates that a response message did not match the provided handler.
  