When hax-engine's fails to parse stdin, make nicer error message

Such an error means either:

• the generated parser is broken (this almost never happens);
• the generated parser is outdated (dune is supposed to regenerate it, but sometimes it doesn't).

The gory details about why the parser failed should be less visible, and we should print a message for the user telling him to reinstall properly things, maybe even to clean the engine folder with dune clean.

Note I tried to do something about that in PR #261, but that was not fully satisfactory.