first, your boss's back ground is not strong and second he is not that support u
that is not a difficult to explain situdation and your boss just need to make a general statment like: well, i understand the situation and felt that is not his fault. (very strong statment) first ...(that reason for the cause), second ...( due to some urgent work, u did not get time to prepare)...third, to be fair, even in school, you need time to prepare your exam......(make it to be a general fact that most people will fail under this situation)...
and your boss need to be strong in making above statment...then no one could use this to against you again....
or next time u could make above explanantion if your boss just hmmmm....but a lot risk lah......ang good luck