Better warning for dbGetQuery() on statement

Current warning doesn't suggest what to do:

``` r
DBI::dbGetQuery(DBI::dbConnect(RSQLite::SQLite()), "CREATE TABLE test (a integer)")
#> Warning in rsqlite_fetch(res@ptr, n = n): Don't need to call dbFetch() for
#> statements, only for queries
#> data frame with 0 columns and 0 rows
```