Use `PhutilMethodNotImplementedException` in `ArcanistExternalLinter`
Use `PhutilMethodNotImplementedException` in `ArcanistExternalLinter`

Summary: Self-explanatory.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11204

julkiewicz/arcanist,julkiewicz/arcanist,julkiewicz/arcanist,julkiewicz/arcanist