Luci GAE Datastore: Singlar Get() and Put() should always return single error
Luci GAE Datastore: Singlar Get() and Put() should always return single error

This covers a few more cases where errors.MultiError might be returned instead of
a singular error

BUG=

Review-Url: https://codereview.chromium.org/2347073002

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