Provide better error when `list` is provided to `dm()`

[IndrajeetPatil]

The current error message is a bit cryptic since users can't know what is def$data:

library(dm)
library(nycflights13)

dm(list(weather, airports))
#> Error in `abort_dm_invalid()`:
#> ! This `dm` is invalid, reason: Not all entries in `def$data` are of class `data.frame` or `tbl_dbi`. Check `dm_get_tables()`.

^{Created on 2022-07-12 by the reprex package (v2.0.1.9000)}