tool: Add flag to avoid catching eval errors

[jtojnar]

We use builtins.tryEval to reduce the chance a check crashes but that comes at a cost of harder debugging, since the function does not tell us anything about the error.

Let’s add a CLI flag that removes this check, to allow observing evaluation errors in their full glory (especially with --show-trace).

[jtojnar]

Merging since I just needed it again.