[vpython] Fix intolerant lookup logic.
[vpython] Fix intolerant lookup logic.

The old probe-based Python lookup will return an error if any single
candidate Python could not be identified. This prevents fallthrough,
e.g., "python2.7" => "python2" => "python" will fail error on
"python2.7" being missing.

Refactor the lookup function a bit to be more tolerant of lookup errors.
It should now try all candidate Python names and only fail if none of
them matched.

BUG=None
TEST=local
R=iannucci@chromium.org, qyearsley@chromium.org

Review-Url: https://codereview.chromium.org/2943513002

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