if len(err) > 0:
                        #
                        # Pass STDERR messages to the user, but do not
                        # raise an error unless the return code was non-zero.
                        #
                        if proc.returncode == 0:
                            message = ("Pip emitted messages on STDERR; these can probably be ignored:\n" +
                                       err)
                            self.log.warning(message)
                        else:
                            message = ("Potentially serious error detected during pip installation:\n" +
                                       err)
                            self.log.critical(message)
                            raise DesiInstallException(message)