Misleading error messages when subscripting homogenous list

[tzakharko]

Subscripting a homogenous list by name throws an error "corrupt <var>: no names". I think it would be better to report something like "<var> has no names". Using the term "corrupt" here may confuse the user into thinking that something went terribly wrong at the R side, where the simple fact is that they have been accessing the object incorrectly (I already had a user reporting a bug because of this message).

Current behavior:

x <- vctrs::list_of(1, 2, 3)
x$aa
#> Error: Corrupt x: no names

Suggested behavior:

x <- vctrs::list_of(1, 2, 3)
x$aa
#> Error: x has no named elements