Fix checking step in `check_blob_exists()`

Currently if no matching blob is found, there's an error message (`no matching {ext} file found`) that should be produced.
However the checking function is incorrect - it uses `check_vec()` inappropriately to check for a character vector.

```r
x <- character(0)
rlang::is_scalar_character(x)
#> [1] FALSE
purrr::every(x, rlang::is_scalar_character)
#> [1] TRUE
```

