need cleaner handling when required env var missing

[barnabyc]

Currently we don't do any sanity checks for the env vars we're looking for, instead we crash, like this:

barnaby:drip 22:18:02 (502) $ nodemon script/init.js
12 Sep 22:18:08 - [nodemon] v0.5.5
12 Sep 22:18:08 - [nodemon] watching: /Users/barnaby/Source/nko2/drip
12 Sep 22:18:08 - [nodemon] running init.js
12 Sep 22:18:08 - [nodemon] starting node

node.js:134
throw e; // process.nextTick error, or 'error' event on first tick
^

TypeError: Cannot read property 'length' of undefined
at Object.urlParse as parse
at Object. (/Users/barnaby/Source/nko2/drip/config/credentials.js:8:19)
at Object. (/Users/barnaby/Source/nko2/drip/config/credentials.js:19:4)
at Module._compile (module.js:402:26)
at Object..js (module.js:408:10)
at Module.load (module.js:334:31)
at Function._load (module.js:293:12)
at require (module.js:346:19)
at Object. (/Users/barnaby/Source/nko2/drip/config/app.js:15:17)
at Object. (/Users/barnaby/Source/nko2/drip/config/app.js:30:4)

12 Sep 22:18:08 - [nodemon] app crashed - waiting for file change before starting...