Better error/warning/info messages

[jpdunc23]

Some (most?) messages show non-user facing helper function calls; e.g., consider the error message from this line:

simChef/tests/testthat/test-experiment.R

Line 762 in b3d71a2

experiment %>% add_vary_across(.dgp = c("DGP2", "DGP1"), z = 1:3)

Error in `private$.check_vary_across()`:
! z: not valid argument(s) to DGP1's dgp_fun
Run `rlang::last_error()` to see where the error occurred.

• Review Error constructors chapter of the Tidyverse design guide