client: removing kvs cache if failed to run cas
client: removing kvs cache if failed to run cas

It seems possible that kvs cache becomes broken. This is to clobber such
broken kvs cache if failed to execute cas client.

Bug: 1202025
Change-Id: I08fa74e795427211b0c476036c9003508f970b00
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2849825
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Reviewed-by: Yoshisato Yanagisawa <5d95c5c75e3d691d9c77888fff4498e04f8b7157@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py