Making EPERM friendlier

An exception is essentially an alternative return value from the function. (The *implementation* of exception handling in languages such as C++ is something quite different, but that is not relevant here). I agree that it is great to have a structured object giving details on the error. However that may be a little ambitious - and in 20 years we might be having arguments about how the error details don't fit into the C structure defined way back in 2013. I suggest that a string, such as Plan 9's errstr, is a reasonable compromise between a single int (clearly inadequate) and a complex structured exception object (too complex to be widely adopted at the kernel level).