improved error messages, fix #74

> seas(AirPassengers, forecast = "dsafdf", forecast.maxback = 5)
Error in mod_spclist(spc, ...) :
  X-13ARIMA-SEATS options should contain a spec and an optional
argument after the dot.
> seas(AirPassengers, x11 = "dsafdf")
Error in mod_spclist(x, x11.save = c("d10", "d11", "d12", "d13", "d16",
 :
  X-13ARIMA-SEATS options should contain a spec and an optional
argument after the dot.