isolate: include which isolate file failed to process in error
isolate: include which isolate file failed to process in error

Bug: 1201013
Change-Id: I0515bb480936c8765893075bbf727f86da833b46
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2843063
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
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