Reversed set assert message for clarity when dealing with long sets a…

[spgarbet]

When dealing with sets with a lot of elements, the error message gets truncated and the most important piece of information to the user is lost. This patch fixes that.

Reproducible example:

data <- data.frame(x = 1:5, X = TRUE)
myset <- c(letters, outer(letters, letters, paste0))
coll <- checkmate::makeAssertCollection()
checkmate::assert_subset(x = names(data), 
                         choices = myset, 
                         add = coll)
checkmate::reportAssertions(coll)

vubiostat/redcapAPI#471