lexpand: misleading warning when dropping exit<entry with fot

[mattirantanen]

Say there is some rows with exit dates before the entry date.
lexpand drops the rows correctly, but when splitting by follow-up (fot) the warning message might be misleading:

The R code below prints a warning
dropped 1 rows where subjects left follow-up before lowest fot breaks value. though a more reasonable warning could be
dropped 1 rows where entry > exit.

DT <- data.table(birth = 2000, entry = 2010, exit = c(2005,2015), status = 1:0)
l <- lexpand(data=DT, birth=birth, entry=entry, exit=exit, 
              status = status, breaks = list(fot = 0:10))

This is quite minor thing, but might help debugging complicated data.