Throw error when config is undefined

[resynth1943]

Closes #42

When a configuration file doesn't export a configuration file as default, ensure the configuration actually exists (it isn't undefined).

[resynth1943]

I've chosen an error because that's more explicit. A || defaultConfig would lead to hard-to-debug errors, and confusing behaviour. Instead, we just throw an error to say "hey, your configuration file is wrong!".

[resynth1943]

We don't do any validation on the import object aside from ensuring it's an object, because IIRC Rollup does runtime validation on the configuration object, and that would flag up any schema errors.