vec_as_names() needs better error message

[hadley]

vctrs::vec_as_names(c("a", "a", "b", "b"), repair = "check_unique")
#> Error: Names must be unique.

^{Created on 2020-03-05 by the reprex package (v0.3.0)}

Suggestion:

vctrs::vec_as_names(c("a", "a", "b", "b"), repair = "check_unique")
#> Error: Names must be unique. Repeated names:
#> * "a" (1, 2)
#> * "b" (3, 4)