Fixes T3386 - Annoying gutter markers in HTML

Summary:
Pamela found these distracting (I agree).
See the task for more info.
We could consider doing this in PJS too

Test Plan:
Open HMTL scratchpad
make mistakes
see no gutter errors

Reviewers: john, brian, pamela

Reviewed By: pamela

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D14736