add more informative error message when `group_by()` is missing

[Aariq]

library(holodeck)
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union

sim_cat(n_obs = 20, n_groups = 2) %>% 
  sim_discr(n_vars = 5, var = 1, cov = 0, group_means = c(-1, 1))
#> Error: Must subset rows with a valid subscript vector.
#> ℹ Logical subscripts must match the size of the indexed input.
#> x Input has size 20 but subscript `!duplicated(x, fromLast = fromLast, ...)` has size 0.

#correct:
sim_cat(n_obs = 20, n_groups = 2) %>% 
  group_by(group) %>% 
  sim_discr(n_vars = 5, var = 1, cov = 0, group_means = c(-1, 1))
#> # A tibble: 20 x 6
#> # Groups:   group [2]
#>    group     V1      V2      V3     V4     V5
#>    <chr>  <dbl>   <dbl>   <dbl>  <dbl>  <dbl>
#>  1 a     -0.247 -0.0864 -1.17   -1.89   0.132
#>  2 a     -2.23  -1.45   -1.87   -0.934 -2.37 
#>  3 a     -1.82  -1.88   -0.108  -0.937 -1.50 
#>  4 a     -2.31  -0.0288 -0.897   1.12  -1.57 
#>  5 a     -1.24  -1.90    0.688  -1.62  -2.83 
#>  6 a     -1.06  -1.29   -1.63    0.409 -0.474
#>  7 a     -0.633 -0.404   0.0105  2.19  -1.79 
#>  8 a     -0.326 -0.0370 -1.17    0.907  0.565
#>  9 a     -0.409 -0.0451  2.04   -2.08  -1.52 
#> 10 a     -0.830 -0.960  -0.874  -0.875 -0.504
#> 11 b      1.42  -0.199   1.43   -0.696  0.531
#> 12 b      0.618  3.50   -0.775   2.34   1.48 
#> 13 b     -0.229  0.559   2.51    0.982  0.384
#> 14 b      1.63   0.682   0.982   1.18   1.20 
#> 15 b      1.51   2.24    1.23    1.57  -0.514
#> 16 b     -1.15   0.601  -0.0712  0.275  2.38 
#> 17 b      0.815  0.661   2.27    1.37   1.24 
#> 18 b      1.70   1.55   -0.344   2.55   1.45 
#> 19 b      0.837  1.36   -0.770   0.505  2.93 
#> 20 b      0.753  1.39    0.469   0.867  3.64

^{Created on 2020-06-04 by the reprex package (v0.3.0)}