ENYO-3061: enforce `ServiceBase#returnFormData()` per-part API

- Each part is expected to be `{ name: "c/yyy", path: "/a/b/c/yyy" }`.
- This change was missing in the early version of this PR.

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>