"In the professed execution of the trusts and powers hereof no trustee shall be liable for any loss to the trust premises arising by reason of any improper investment made in good faith or for the negligence or fraud of any agent employed by him or by any other trustee hereof although the employment of such agent was not strictly necessary or expedient or by reason of any other matter or thing except wilful and individual fraud or wrongdoing on the part of the trustee who is sought to be made liable."