Raise a dedicated NotFoundError if experiment or work unit are not found.
Raise a dedicated NotFoundError if experiment or work unit are not found.

PiperOrigin-RevId: 397730427
Change-Id: I84a019ca11ed5301d34fead5d2587faa33eafa88
GitOrigin-RevId: 709130cf1bd496eb0cd22c24214521967d09eb3d

deepmind/xmanager,deepmind/xmanager