client: print more details about cache overflow failure
client: print more details about cache overflow failure

R=7cdab2cfab351f23814786ba39716e90eed69047@chromium.org

Change-Id: I52e2701e46f68803b8a9e6ccd4a2fc0234e4fb75
Reviewed-on: https://chromium-review.googlesource.com/881225
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py