xso-proc: add support for child elements

xso-proc/src/error_message.rs

@@ -79,3 +79,11 @@ pub(super) fn on_missing_attribute(parent_name: &ParentRef, field: &Member) -> S
         parent_name
     )
 }
+
+/// Create a string error message for a missing child element.
+///
+/// `parent_name` should point at the compound which is being parsed and
+/// `field` should be the field to which the child belongs.
+pub(super) fn on_missing_child(parent_name: &ParentRef, field: &Member) -> String {
+    format!("Missing child {} in {}.", FieldName(&field), parent_name)
+}