Handle shadowlisp panics better

[burke]

There are lots of ways to generate a ketos->rust panic, and this shows in the terminal like:

~/s/g/S/shadowenv 𝒎░% cd ~[//areas/core/shopify]
thread 'main' panicked at src/lang.rs:75:69:
called `Result::unwrap()` on an `Err` value: TypeError { expected: "string", found: "unit", value: Some(Unit) }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
//a/c/shopify 𝒎 ≟░%

We should catch these and thoughtfully print them as 1-2 lines shadowenv errors instead.