Appends the given exception at the top of the exception list and delete the oldest one (bottom most).
        :param stacktrace: Stacktrace string as returned by 'traceback.format_exc()' in an 'except' block.

def append_exception(self,
                         stacktrace):
        """
        Appends the given exception at the top of the exception list and delete the oldest one (bottom most).
        :param stacktrace: Stacktrace string as returned by 'traceback.format_exc()' in an 'except' block.
        """
        # Delete the last message of the list
        if len(self.exceptions) > 0:
            del self.exceptions[-1]

        # Append the new message at the top
        self.exceptions.insert(0, stacktrace)
        self.changes_made = True

        # Redraw
        self.redraw()