[R] Mixed-type list columns fail with unintuitive error message

### Describe the bug, including details regarding any error messages, version, and platform.

Often surfaced when users are trying to call `write_parquet()` on tibbles with list-columns

``` r
library(arrow)

# works
df <- tibble::tibble(x = list(NULL, 1))
arrow_table(df)
#> Table
#> 2 rows x 1 columns
#> $x: list>

# fails
df <- tibble::tibble(x = list(NULL, 1, "a"))
arrow_table(df)
#> Error: Invalid: cannot convert

# fails
df <- tibble::tibble(x = list(1, "a"))
arrow_table(df)
#> Error: Invalid: cannot convert
```

### Component(s)

R