Step 4: Handle pop's error case
Pop errors when the stack is empty. Write a StackEmpty class to represent this error at the top of the exceptions.cpp file. This exception should take a string in its constructor and have a GetMessage() accessor for this string. throw this exception with an error message passed to the exception's constructor, instead of printing an error message and doing exit(1). Write the code in main to catch the exception and use cerr to print out what GetMessage() says.