make fixes based on code climate suggestions

[ashleygwilliams]

https://codeclimate.com/github/npm/marky-markdown/issues/categories/bugrisk

[zeke]

I'm not sure about the suggestion to use throw instead of process.exit(). Doing so will result in a bunch error output to the screen, when all we really want is usage:

❯ node bin/marky-markdown.js
/Users/z/zeke/marky-markdown/bin/marky-markdown.js:7
  throw new Error('Usage:\n\nmarky-markdown some.md > some.html')
  ^

Error: Usage:

marky-markdown some.md > some.html
  at Object.<anonymous> (/Users/z/zeke/marky-markdown/bin/marky-markdown.js:7:9)
  at Module._compile (module.js:398:26)
  at Object.Module._extensions..js (module.js:405:10)
  at Module.load (module.js:344:32)
  at Function.Module._load (module.js:301:12)
  at Function.Module.runMain (module.js:430:10)
  at startup (node.js:141:18)
  at node.js:980:3

https://twitter.com/zeke/status/684425698321158144

[ashleygwilliams]

closing. i don't think these checks are worthwhile.