show messsage if invalid dir name on createDir

[tgfjt]

Hi, Dat people!

I saw an error with wrong argument to clone sample.
How do you like to show a message to get valid argument?

$ dat clone dat://xxxx 2017
fs.js:288
  binding.access(pathModule._makeLong(path), mode, req);
          ^

TypeError: path must be a string or Buffer
    at Object.fs.access (fs.js:288:11)
    at createDir (/Users/tgfjt/workspace/dat/src/commands/clone.js:95:10)
    at /Users/tgfjt/workspace/dat/src/commands/clone.js:84:7
    at /Users/tgfjt/workspace/dat/node_modules/dat-link-resolve/index.js:34:23
    at /Users/tgfjt/workspace/dat/node_modules/call-me-maybe/index.js:11:28
    at _combinedTickCallback (internal/process/next_tick.js:95:7)
    at process._tickCallback (internal/process/next_tick.js:161:9)
    at Function.Module.runMain (module.js:607:11)
    at startup (bootstrap_node.js:158:16)
    at bootstrap_node.js:575:3

[martinheidegger]

LGTM - The build errors might be fixed by re-running the travis build

[joehand]

Thanks for fixing it and you patience =)

[tgfjt]

thanks, hooray! 🎉