Step 4: Handle pop's error case

An error occurs if pop() is called when the stack is empty. At the top of exceptions.cpp, write a StackEmpty class to represent this error. This exception should take a string in its constructor and have a GetMessage() accessor that returns the string. Throw this exception with an error message passed to the exception's constructor rather than printing an error message and calling exit(1). Write the code in main() to catch the exception and use cerr to print out the message returned by GetMessage().