message: Fix string formatting and rephrase error for JWT MAC factory
diff: @@ -26,11 +26,11 @@ import (
  // NewMAC generates a new instance of the JWT MAC primitive.
  func NewMAC(h *keyset.Handle) (MAC, error) {
  if h == nil {
  - return nil, fmt.Errorf("key handle can't be nil")
  + return nil, fmt.Errorf("keyset handle can't be nil")
  }
  ps, err := h.PrimitivesWithKeyManager(nil)
  if err != nil {
  - return nil, fmt.Errorf("jwt_mac_factory: cannot obtain primitive set: %s", err)
  + return nil, fmt.Errorf("jwt_mac_factory: cannot obtain primitive set: %v", err)
  }
  return newWrappedJWTMAC(ps)
  }
  