Add rb_sys_fail for checking if some syscall fail
Add rb_sys_fail write(2)
Revert "Add rb_sys_fail write(2)"

This reverts commit 6765a1b9494be3313766923d10e68554b28a4be4.
Remove unnecessary assert(0)
Merge pull request #23 from kubo39/rb_sys_fail

Using rb_sys_fail for some syscalls fail