survtab_ag: uninformative error message with cif.obs

[mattirantanen]

I tried to calculate cumulative risk, i.e. 1 - observed survival using the following code. As expected it didn't work since only one death cause was given in d, but the error message was not that informative.
I also suggest adding an example of cif.obs in the examples section of help page.

BL <- list(fot= seq(0,19,1/12), per=c(2008,2013))
sire2 <- sire[dg_date<ex_date, ]
sire2$statusf <- factor(sire2$status, levels = 0:2, 
                        labels = c("alive", "canD", "othD"))

x <- lexpand(sire2, 
             birth  = bi_date, entry = dg_date, exit = ex_date,
             status = statusf,
             breaks=BL, aggre = list(fot))
x
st <- survtab_ag(fot ~ 1, data = x, d = 'fromalivetocanD', surv.type="cif.obs")

The error message:

 Error in get(d.var) : object 'd.d' not found