Unbreakable average program
First, use try/catch blocks around parts that can throw exceptions.  For example, what happens when an IOException is thrown by the file, perhaps because the data is corrupted and the file exists, but cannot be opened?

Second, I don't know what constitutes failure, but an app-closing failure is failure in my book:

if(!file.isFile()){
    System.err.println("ERROR: File does not exist.");
    System.exit(-1);
}


Third, @SirPython is right - use the built-in method for reading your number.  There are built-in functions provided for reading both doubles and ints, whichever your program needs.

Fourth, this will not necessarily help robustness, split your method logic up.  You are doing a lot of printing in your fileInput() method.  Do your printing in a dedicated method.  Do your input in a dedicated method.  Feel free to throw an exception when exceptional behavior happens, but be sure to handle the exception.  Split your logic up into bite-sized components and implement them separately - this will make reuse easy.

Fifth, recover from your errors.  If you need, start the entire input processes over, but do not close the program.  Closing the program because of an error will probably be counted as an error, and is unacceptable in many programs.