Export `null` when not found

I'm on Linux and I don't have Firefox installed (shocking I'm sure).

Trying to run smokestack causes an _Error_ to be thrown by `which`:

``` bash
$ browserify test/index.js | smokestack -b chrome | faucet

/home/kenan/.npm-packages/lib/node_modules/smokestack/node_modules/firefox-location/node_modules/which/which.js:78
  throw new Error("not found: "+cmd)
        ^
Error: not found: firefox
    at Function.whichSync [as sync] (/home/kenan/.npm-packages/lib/node_modules/smokestack/node_modules/firefox-location/node_modules/which/which.js:78:9)
    at Object.<anonymous> (/home/kenan/.npm-packages/lib/node_modules/smokestack/node_modules/firefox-location/index.js:7:37)
    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)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/home/kenan/.npm-packages/lib/node_modules/smokestack/index.js:4:16)
    at Module._compile (module.js:456:26)
not ok 1 no plan found
not ok 2 no assertions found
⨯ fail  2
```

So this error needs to be caught by either this module or smokestack itself when it does `require('firefox-location')`.