Better error message for the one_of selector

[jjchern]

Example:

library(dplyr)
df = data_frame(a = 1, b = 2)

varlist1 = c("c")
select(df, one_of(varlist1))

varlist2 = c("a", "c")
select(df, one_of(varlist2))

The first selection returns the whole data frame, but I expect seeing an error or message indicating that there's no variable called c.

The second selection returns an error:

Error: Each argument must yield either positive or negative integers

This error message is not quite clear to me. It would be great if the second selection yields column a with a warning saying that c can not be found.