Refactor errors

crates/pile-value/src/extract/traits.rs

@@ -7,6 +7,12 @@ pub trait ObjectExtractor: Send + Sync {
 	/// Get the field at `name` from `item`.
 	/// - returns `None` if `name` is not a valid field
 	/// - returns `Some(Null)` if `name` is not available
+	///
+	/// For extractors that parse binary, this fn should return
+	/// an error only if we failed to obtain the data we need (permission denied, etc).
+	///
+	/// If the underlying data has an invalid format (e.g, running a pdf extractor on a non-pdf file),
+	/// this fn should return `Ok(Some(None))`.
 	async fn field(
 		&self,
 		name: &pile_config::Label,