cache: make error more verbose
cache: make error more verbose

This is for tasks like
https://chromium-swarm-dev.appspot.com/task?id=48df82b368a6b410

Bug: 932396
Change-Id: I92e45c8bfc581074e7bd687b4bb1c35ffbfdd4d1
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1943832
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>

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