Better error/warning/info messages

[jpdunc23]

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Some (most?) messages show non-user facing helper function calls; e.g., consider the error message from this line: https://github.com/Yu-Group/simChef/blob/b3d71a2d5594eab4f8f8ec1006aba8e723508b68/tests/testthat/test-experiment.R#L762

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