downloader: retry fetch if it failed
downloader: retry fetch if it failed

This is mitigation for failure like
https://chromium-swarm.appspot.com/task?id=490c7d1721223710

network connection is not so reliable.

Bug: 932396
Change-Id: I97827f8a9c819529b80564993d2f6b032982bfe2
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1958395
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
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