Shift layers don't throw intuitive errors when trying to use distinct counts

[mstackhouse]

Prerequisites

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Description

Shift layer errors

Steps to Reproduce (Bug Report Only)

library(Tplyr)

tplyr_table(mtcars, carb) %>% 
  add_layer(
    group_shift(vars(column=vs, row=am)) %>% 
      set_distinct_by(cyl) %>% 
      set_format_strings(
        f_str('xx | xx | xx.x% || xx | xx | xx.x%', n, total, pct, distinct_n, distinct_total, distinct_pct)
      )
  ) %>% 
  build()
#> Error in `mutate()`:
#> ! Problem while computing `n = construct_shift_string(...)`.
#> ℹ The error occurred in group 1: carb = "1".
#> Caused by error in `if (nchar(x)[1] < left_pad) ...`:
#> ! missing value where TRUE/FALSE needed

^{Created on 2022-07-19 by the reprex package (v0.3.0)}

Expected behavior: [What you expected to happen]

A graceful error indicating that distinct counts aren't supported for shift layers

Actual behavior: [What actually happened]

Error message from failing internals

Versions

This is using current devel branch