lucictx: include json in error
lucictx: include json in error

This is to investigate failure of
https://ci.chromium.org/ui/p/infra/builders/try/infra-try-bionic-64/b8845513009289811088/overview

It doesn't reproduce in my local env strangely.

Bug: 1207896
Change-Id: Idaa01ed2649c9b12c72f50137277ce7d82720d41
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2936164
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
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