avoid panic when using from nuon (#7533)

# Description

Fixes #5996 

Just found a relative easy way to fix the issue

# User-Facing Changes

```
❯ open $nu.plugin-path | from nuon
Error:
  × error when loading nuon text
   ╭─[entry #36:1:1]
 1 │ open $nu.plugin-path | from nuon
   ·                        ────┬────
   ·                            ╰── could not load nuon text
   ╰────

Error:
  × Error when loading


❯ open $nu.config-path | from nuon
Error:
  × error when loading nuon text
   ╭─[entry #37:1:1]
 1 │ open $nu.config-path | from nuon
   ·                        ────┬────
   ·                            ╰── could not load nuon text
   ╰────

Error:
  × error when loading
```

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/tests/format_conversions/nuon.rs

@@ -340,6 +340,14 @@ fn quotes_some_strings_necessarily() {
     assert_eq!(actual.out, "list<string>");
 }
 
+#[test]
+fn read_code_should_fail_rather_than_panic() {
+    let actual = nu!(cwd: "tests/fixtures/formats", pipeline(
+        r#"open code.nu | from nuon"#
+    ));
+    assert!(actual.err.contains("error when parsing"))
+}
+
 proptest! {
     #[test]
     fn to_nuon_from_nuon(c: char) {