ww_multi_scale() should give friendlier errors when arguments are missing

[mikemahoney218]

waywiser::ww_multi_scale(waywiser::ny_trees, n = 2)
#> Error in `data[[truth_var]]` at waywiser/R/multi_scale.R:383:2:
#> ! Can't extract column with `truth_var`.
#> ✖ Subscript `truth_var` must be size 1, not 0.
#> Backtrace:
#>      ▆
#>   1. ├─waywiser::ww_multi_scale(waywiser::ny_trees, n = 2)
#>   2. └─waywiser:::ww_multi_scale.sf(waywiser::ny_trees, n = 2) at waywiser/R/multi_scale.R:158:4
#>   3.   ├─data[[truth_var]] at waywiser/R/multi_scale.R:383:2
#>   4.   └─tibble:::`[[.tbl_df`(data, truth_var) at waywiser/R/multi_scale.R:383:2
#>   5.     └─tibble:::tbl_subset2(x, j = i, j_arg = substitute(i))
#>   6.       └─tibble:::vectbl_as_col_location2(...)
#>   7.         ├─tibble:::subclass_col_index_errors(...)
#>   8.         │ └─base::withCallingHandlers(...)
#>   9.         └─vctrs::vec_as_location2(j, n, names, call = call)
#>  10.           └─vctrs:::result_get(...)
#>  11.             └─rlang::cnd_signal(x$err)
waywiser::ww_multi_scale(waywiser::ny_trees, agb, n = 2)
#> Error in `data[[estimate_var]]` at waywiser/R/multi_scale.R:387:2:
#> ! Can't extract column with `estimate_var`.
#> ✖ Subscript `estimate_var` must be size 1, not 0.
#> Backtrace:
#>      ▆
#>   1. ├─waywiser::ww_multi_scale(waywiser::ny_trees, agb, n = 2)
#>   2. └─waywiser:::ww_multi_scale.sf(waywiser::ny_trees, agb, n = 2) at waywiser/R/multi_scale.R:158:4
#>   3.   ├─data[[estimate_var]] at waywiser/R/multi_scale.R:387:2
#>   4.   └─tibble:::`[[.tbl_df`(data, estimate_var) at waywiser/R/multi_scale.R:387:2
#>   5.     └─tibble:::tbl_subset2(x, j = i, j_arg = substitute(i))
#>   6.       └─tibble:::vectbl_as_col_location2(...)
#>   7.         ├─tibble:::subclass_col_index_errors(...)
#>   8.         │ └─base::withCallingHandlers(...)
#>   9.         └─vctrs::vec_as_location2(j, n, names, call = call)
#>  10.           └─vctrs:::result_get(...)
#>  11.             └─rlang::cnd_signal(x$err)

^{Created on 2023-06-14 with reprex v2.0.2}