Fix linter config check

Summary: Fixes T6311 and T5124 by returning all configured linters from buildLinter(), and making ArcanistExternalLinter::checkBinaryConfiguration() not crash if there's no executable to run.

Test Plan: arc linters in rP shows "Configured" and "ERROR" as appropriate; Adding a broken linter to .arclint in rARC doesn't invoke it's not actually needed, and prints error if it is.

Reviewers: joshuaspence, epriestley, Blessed Reviewers

Reviewed By: joshuaspence, epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T6311

Differential Revision: https://secure.phabricator.com/D10773