Improve config checks at startup time

[jamesaoverton]

We want to make our lives easier. When there's a problem we need to debug, how can we make it easy to find and isolate the problem. This may require more discussion.

We're already checking some aspects of the config at startup, but I'd like to implement a comprehensive set of checks:

• ERROR if any required keys are missing
• WARN if unexpected keys are found
• ERROR if a key we use has an invalid value

We also discussed outputting the active config options, the key difference being dev/test/prod.

The core of this would be a function, but we could potentially "access" it in multiple ways:

1. just print to log or STDOUT or STDERR
2. a special command-line option
3. a separate command
4. an HTML page (once DROID is running)