cache: include cache path in error
cache: include cache path in error

This is for https://crbug.com/1116671#c6

Bug: 1116671
Change-Id: I067112e65c6897c52aee88d74e8116659ca7997b
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2359056
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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