Engine: when the parsing of JSON fails, print an helpful message

[W95Psp]

Currently, if you run hax on a crate on which you run an older hax before, you might get a parsing error.
The parsing error is currently a stack trace from OCaml, we should instead add a message that tells the user this is probably:

• either hax frontend / engine mismatch
• either cache issue (so needs cargo clean)