More informative error for non-unique item names

[jennybc]

If possible, it would be great if this error told me which names R6 thinks are not unique.

> devtools::load_all(".")
ℹ Loading gargle
Error in `load_all()`:
! Failed to load R/AuthState-class.R
Caused by error in `R6::R6Class()`:
! All items in public, private, and active must have unique names.

[maciekbanas]

I just wanted to create same issue, but here it goes...