Error codes in fluffy.c & .h, some minor fix

[Neol-d2022]

Hi all, I have done some modifications (most of them is about error code reporting), please review them.

Also, at the end of the function static int fluffy_handoff_event() in fluffy.c, it returns whatever client has returned. Note that the value returned by clients could accidentally collide with error codes (recently) defined in fluffy.h.