more strict nuon handling, better nuon errors (#4576)

* more strict nuon handling, better nuon errors

* Improve errors a bit more

crates/nu-cli/src/errors.rs

@@ -41,6 +41,14 @@ impl<'src> miette::Diagnostic for CliError<'src> {
 
     // Finally, we redirect the source_code method to our own source.
     fn source_code(&self) -> Option<&dyn SourceCode> {
-        Some(&self.1)
+        if let Some(source_code) = self.0.source_code() {
+            Some(source_code)
+        } else {
+            Some(&self.1)
+        }
+    }
+
+    fn related<'a>(&'a self) -> Option<Box<dyn Iterator<Item = &'a dyn miette::Diagnostic> + 'a>> {
+        self.0.related()
     }
 }