[lucicfg] Print the service being validated
[lucicfg] Print the service being validated

Otherwise when changing multiple services at once it may be hard to know
which one is misconfigured.

R=9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Change-Id: Ide8fe174848cc1ccee570346e1434f27a741a4ba
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1857039
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go