isolate: verbose error
isolate: verbose error

This is for more clear error around
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket/8829866703211453793/+/u/isolate_tests__with_patch_/stdout

Change-Id: I78985f0cc25061887c7586ec0b602192c780049d
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3298539
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Fumitoshi Ukai <0e88e065dd7f7d7f11da28b068df91c65342563c@google.com>
Reviewed-by: Fumitoshi Ukai <0e88e065dd7f7d7f11da28b068df91c65342563c@google.com>

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