Move linter exception classes to src/lint/linter/exception

Summary: The src/lint/linter directory is a bit cluttered at the moment.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11309