[resultdb] fix ExitCodeCommandFailure
[resultdb] fix ExitCodeCommandFailure

Linux uses the lower 8 bits of a process state to represent
the exit code. Therefore, if an rdb execution fails, the exit code is
actually 1001 & 255.

https://golang.org/pkg/os/#Exit states that the exit code should be
in the range [0, 125].

R=iannucci@chromium.org
CC=332b6c56634d0f63001046c664a2370006a02420@chromium.org,ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org

Change-Id: I4607bcbfb1bda297b4a051be0f62d296a1e02b5d
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2369499
Reviewed-by: Chan Li <332b6c56634d0f63001046c664a2370006a02420@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

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