[isolatedclient] Mark io.Copy errors retriable
[isolatedclient] Mark io.Copy errors retriable

Bug: 1064012
Change-Id: I6d6f777b75ba2bde27f77f9f70b7935f536ea8fc
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2124444
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>

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