When box initialization fails (e.g., image pull failure), the cleanup
guard removes the box from DB. If stop() is then called (e.g., from a
finally block), it would try to save to a non-existent box, returning
NotFound which masked the original error.

Changes:
- box_impl.rs: Handle NotFound gracefully in stop() - if box was already
  removed by cleanup, return Ok(()) instead of propagating the error
- Remove redundant "Box not found:" prefix from NotFound error messages
  since #[error("box not found: {0}")] already adds this prefix

Before: "box not found: Box not found: abc123"
After:  "box not found: abc123"

And most importantly, users now see the actual error:
"storage error: Failed to pull image 'xxx' after trying 1 registry"
instead of the misleading "box not found" error.