Closes the current functional group by writing its footer segment.
The CreateFunctionalGroupFooter method will write any existing segments to the output stream and then generate and write the functional group closing segment (GE or UNE). If there is an open transaction set, CreateFunctionalGroupFooter will first call CreateTransactionFooter to close it. If there is no functional group open, an exception will be thrown.
This method returns a result code; 0 indicates success, while a non-zero error code indicates that this method encountered an error during its execution. If an error occurs, the GetLastError() method can be called to retrieve the associated error message. (Note: This method's result code can also be obtained by calling the GetLastErrorCode() method after it returns.)