Throw classed error if allow_empty = FALSE + add more context to error message by adding error_arg to eval_select()

[olivroy]

Closes #347

basically implement what was raised in #282 (comment)

Addresses part of #327

I have to say this fix is a bit naive and narrow and lacks a more general view.

pkgload::load_all() 
#> ℹ Loading tidyselect
# give a better error message if you don't want empty selection
 # if supplying `error_arg`
 select_not_empty <- function(x, cols) {
   eval_select(expr = enquo(cols), data = x, allow_empty = FALSE, error_arg = "cols")
 }
 try(select_not_empty(mtcars, cols = starts_with("vs2")))
#> Error in select_not_empty(mtcars, cols = starts_with("vs2")) : 
#>   `cols` must select at least one column.

^{Created on 2024-04-25 with reprex v2.1.0}

Would it make sense to recognize it in eval_relocate()?

A version I had locally had error_arg = c(before_arg, after_arg), but couldn't tell if it was useful.

This implementation doesn't change the default, which mean it is opt-in.

Reason behind.

I was working on gt and discovered allow_empty and I thought I'd use it to throw messages, (without having to rethrow it again)

But I found that the error message Can't select empty items was not informative at all, so it is required to rethrow it or make tweaks in tidyselect.

So I thought it would be great if tidyselect had a mechanism to do this, and discovered it was proposed last time tidyselect was updated, but was never implemented.