[vim3] fix `getvar partition-size:userdata`

We partially support the "userdata" partition by allowing erase to be a
no-op for the `fastboot -w` flag, but it doesn't actually exist on disk
so querying `partition-size:userdata` hit a buggy code path.

This code path is incorrect anyway, when a partition cannot be found we
need to return a fastboot FAIL message, but the code was printing an
error message inside the OKAY reponse instead which tricks the caller
into thinking the contents should be a partition size.

Bug: b/327961098
Change-Id: I46403152efcc12c617f5811674f438d4dab59323
Reviewed-on: https://turquoise-internal-review.googlesource.com/c/third_party/u-boot/+/815490
Reviewed-by: Yecheng Zhao <zyecheng@google.com>
GitOrigin-RevId: 642f58a81710552dd0d06e650e6a1bc330a6c4fd