client: show exception when failed to run command
client: show exception when failed to run command

Error message with command is sometimes not sufficient to explain why
execution failed.
https://chromium-swarm-staging.appspot.com/task?d=true&id=4b097e0bc9c8d310

Bug: b/144318226
Change-Id: Id062178506104f00c56428c159e01a40f5941e67
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2105532
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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