[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
Committed: https://github.com/luci/luci-go/commit/d9f6561f4b6d29a16f889ad4f6f2898c5794303a

[iannucci, 2017-06-15 19:00:17]

Description was changed from

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

The new 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
==========

to

==========
[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
==========

[iannucci, 2017-06-15 19:02:05]

The CQ bit was checked by iannucci@chromium.org

[iannucci, 2017-06-15 19:02:06]

lgtm

[commit-bot: I haz the power, 2017-06-15 19:02:15]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-06-15 19:10:00]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1497553325745550, "parent_rev":
"35426aab5e558df793b116a5f0c516633a32376b", "commit_rev":
"d9f6561f4b6d29a16f889ad4f6f2898c5794303a"}

[commit-bot: I haz the power, 2017-06-15 19:10:02]

Description was changed from

==========
[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
==========

to

==========
[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
Committed:
https://github.com/luci/luci-go/commit/d9f6561f4b6d29a16f889ad4f6f2898c5794303a
==========

[commit-bot: I haz the power, 2017-06-15 19:10:04]

Committed patchset #1 (id:1) as
https://github.com/luci/luci-go/commit/d9f6561f4b6d29a16f889ad4f6f2898c5794303a

[qyearsley, 2017-06-15 20:13:58]

On 2017/06/15 19:10:04, commit-bot: I haz the power wrote:
> Committed patchset #1 (id:1) as
>
https://github.com/luci/luci-go/commit/d9f6561f4b6d29a16f889ad4f6f2898c5794303a

LGTM

Does TEST=local here mean that you ran vpython locally without `python2.7` in
the path to check that it found the right python binary?