Don't use error context from JSHint in linter messages

Summary:
The error context (a.k.a. "original text") as provided by jshint is not very useful and actually causes arc lint to display the lint message incorrectly.

The underlying problem here is that the error context from jshint contains the entire line from the input file rather than just the offending source code.

Test Plan: Ran arc lint -- webroot/rsrc/js/core/behavior-hovercard.js (in rP) and verified that the output looked reasonable.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9675