The master or employer shall not be liable under any of the provisions of section 44-1401 where the injury to the employee was caused by the incompetency of a coemployee, and such incompetency was known to the employee injured, and the employee injured failed within a reasonable time to give notice thereof to the master or employer, or to some person superior to himself engaged in the service or employment of the master or employer who had entrusted to him some general superintendence, unless the master or employer or such superior already knew of such incompetency of such coemployee, and such master or employer failed or refused to discharge such incompetent employee or failed or refused to investigate the alleged incompetency of the coemployee and discharge him if found incompetent.