Demote bootstrap token validation error to warning level.
Demote bootstrap token validation error to warning level.

Security scanner likes to spam the server with bad tokens. No need to send
ereporter alert, it's no big deal.

R=maruel@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2072883002

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