Function fz_error_callback 

pub unsafe extern "C" fn fz_error_callback(
    ctx: *mut fz_context,
    user: *mut *mut c_void,
) -> fz_error_cb

Retrieve the currently set error callback, or NULL if none
has been set. Optionally, if user is non-NULL, the user pointer
given when the warning callback was set is also passed back to
the caller.