Convert `Stop()` function to use variable length arguments

It may provide users additional information if the `Stop()` function can provide variable length arguments. We could then fold the more informative console errors (`pr_err` and `pr_warn`) into the output of the graphical error messages.  See #47 for detail on one such example.

ref: https://www.tutorialspoint.com/c_standard_library/c_function_vsprintf.htm