Develop a Java program that takes a user-specified input string as a command line argument and finds the last occurring character in it. The program should be able to handle strings of any length and return the index of the last occurring character. The program should also output an error message if the input string is empty or contains non-alphabetic characters. The program should be written using object-oriented programming principles and must be well-documented with comments. It should also include error handling and appropriate test cases to ensure correct functionality.
This prompt doesn't seem malicious.