Fix JSON parse failure error message.
Fix JSON parse failure error message.

Bug: 1051371
Change-Id: If45f8901a687d042d5bf89e605d333a2da1131f5
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2086413
Auto-Submit: Owen Rodley <ac3ca500c35ad8417cba974d309c69594f145e39@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

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