add error tracking

[CharString]

Inspiration sources

• domdomegg/ottie by the author of the implementations of W and M that we're using.
• Explaining type inference — Duggan et al. for caveats of naively recording explanation paths
• Possibly the Elm type checker; it's a simple language and it gives excellent actionable feedback on type errors.

Todo

• Treat partial applications e.g. {"<": [1]} as errors (see treatment of {"var": []})
• Throw TypeError with helpful text and explanation data like paths in the expression so the location can be marked in the source text
• Error cases marked "TODO" in test-d/tests.json