Improve config validation

[lawrencejones]

The errors are a bit messy. If you fail to parse the file, we print the default template output, which is confusing.

If you use keys that aren't recognised, we'll accept them: this is particularly confusing.

[lawrencejones]

Detection of duplicate type names would be great too.

[lawrencejones]

#65 will fix the unrecognised key problem, but several things still left to improve:

• General output: it's hard to see the error and the pretty-printing of the config isn't helping anyone.
• Duplicate type names.
• Other logical config errors.