shell.mv does not gracefully handle file permission errors

[nfischer]

Originally reported here: #64 (comment) (/CC @akras14)

var shell = require("shelljs");
shell.mv("test", "/");

$: node test.js 
shell.js: internal error
Error: EACCES, permission denied 'test'
    at Object.fs.renameSync (fs.js:554:18)
    at /home/alexk/projects/support/uploader/node_modules/shelljs/src/mv.js:77:8
    at Array.forEach (native)
    at Object._mv (/home/alexk/projects/support/uploader/node_modules/shelljs/src/mv.js:53:11)
    at Object.mv (/home/alexk/projects/support/uploader/node_modules/shelljs/src/common.js:186:23)
    at Object.<anonymous> (/home/alexk/projects/support/uploader/test.js:3:7)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)

---

Basic issue is that / is owned by root, and so the user doesn't have permission to write to that folder.