Write a program called `rn`. The objective is to convert a number, given as an argument, into a roman number and print it with its roman number calculation.
The program should have a limit of `4000`. In case of an invalid number, for example `"hello"` or `0` the program should print `ERROR: can not convert to roman number`.
The program should have a limit of `4000`. In case of an invalid number, for example `"hello"` or `0` the program should print `ERROR: can not convert to roman digit`.