handle permission error

[yoichi]

Current behavior

slotchmod spins the slot, even though it's certain that chmod will fail.

vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ mkdir d
vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ touch d/f
vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ chmod -x d
vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ ./slotchmod d/f
chmod 0555 d/f
panic: chmod d/f: permission denied

goroutine 1 [running]:
main.changeMode(0xc00001c0c0, {0xc000010050, 0x1, 0x6})
	/home/vagrant/ghq/github.com/jiro4989/slotchmod/main.go:86 +0x2e5
main.main()
	/home/vagrant/ghq/github.com/jiro4989/slotchmod/main.go:42 +0x155

Expected behavior

Like the case with the missing file, I think it is good to make an error first.

Behavior after the change

minimum permission:

vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ chmod 100 d
vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ ./slotchmod d/f
chmod 0026 d/f
vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ chmod 000 d/f
vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ echo $?
0

not enough permission:

vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ chmod 600 d
vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ ./slotchmod d/f
[ERR] d/f permission denied.
vagrant@vagrant:~/ghq/github.com/jiro4989/slotchmod$ chmod 000 d/f
chmod: cannot access 'd/f': Permission denied

[jiro4989]

awesome!