Better error messages

On a malformed JSON file:

```
: mgree@rocinante:~/talks/20211025plos [master] ; ~/ffs/target/release/ffs -i demo.json 
thread 'main' panicked at 'JSON: Error("expected value", line: 7, column: 12)', src/format.rs:322:45
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```