Flushes the parser and checks its end state.
When Flush is called, the parser flushes all its buffers, firing events as necessary, and then checks its end state.
Any extra un-parsed data will be populated in the ExtraData property after this method is called.
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.)