Make LazyMultiError.Assign return bool.
Make LazyMultiError.Assign return bool.

This allows lme.Assign to be easily incorporated into conditional
statements which need to determine if err was nil or not.

R=vadimsh@chromium.org, dnj@chromium.org, estaab@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1253083004 .

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