Throw error if required params are not specified in config.json.

Fixes #3.

Note that this just checks for a few missing params in config.json. The other params are already checked in the various places that use them (e.g., s3storage.js), and if the entire config.json or cert files are missing, errors will already be thrown by the existing code that tries to read them. (Could make these friendlier but they seem pretty obvious.)

@dangoor