New custom constraint and a callback fix

Summary:
Added a new custom constraint for isValidColor, now that custom constraints work.
I'll use it in a few challenges (I don't actually use notDefaultColor, since we don't
actually do a default color, so I could just delete that now).

I also reworded the Oh Noes messages to be more in the PJS style.
I was getting confused by their wording myself, as to whether they were telling me
what *should* be the case or what *was* the case.
I imagine I'll keep iterating as I see more challenge feedback come in.

Test Plan:
- Run live-editor tests
- Make a few errors while writing HTML, see the new messages

Reviewers: alexrodrigues, john

Reviewed By: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D14701