Make `arc shell-complete` more robust.

Summary: Fixes T5122. Add some basic error handling to the `shell-complete` workflow.

Test Plan: `arc shell-complete` no longer throws errors.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5122

Differential Revision: https://secure.phabricator.com/D9268
Add an `ArcanistGoLintLinter`.

Summary: Create bindings for `golint` as an external linter.

Test Plan: Wrote and executed unit tests.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9246
Fix diffusion.looksoon call to actually work

Summary:
This is all derped up, and we ignore the error so old installs don't break.

Underp it.

Test Plan: Did `arc land`, verified that the call actually went through successfully.

Reviewers: btrahan, davedash

Reviewed By: davedash

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9271
Merge branch 'master' of github.com:facebook/arcanist