Syntax error panic (`Utf8Error`)

`test.ipso`:
```
test : ()
test = 
  comp
    let prefix = if x then "📁 " else "📄 " in
```

Output:
```
$ ipso test.ipso
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Utf8Error { valid_up_to: 41, error_len: None }', src/lib.rs:223:61
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```