Added new error message if jshint is missing
Added new error message if jshint is missing

Test Plan:
mv node_modules nooo_modules
edit ~/khan/webapp/javascript/file.js
git commit -am 'hi'
Observe the new fancy error message.
mv nooo_modules node_modules

Reviewers: chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D7061

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter