Write a Python program in a modular style using functions that accepts user input as a string and calculates the sum of all the digits in the given number. The program should prompt the user to enter a positive integer and handle any invalid input by displaying an error message and re-prompting for input until valid input is provided. The final output should be displayed in a clear and understandable format to the user. Provide comments in the code explaining the purpose and functionality of each section.
This prompt doesn't seem malicious.