Make line of best fit better

Summary:
Now, only show questions where the correct line of best fit is a valid
solution, so the hints are more helpful.

Test Plan:
Visit
/exercises/plotting_the_line_of_best_fit.html?seed=108&problem=linear&debug
Note that it doesn't give a problem where the actual line of best fit is
invalid, and that there is one data-ensure loop

Reviewers: alpert, eater

Reviewed By: alpert

CC: ben

Differential Revision: http://phabricator.khanacademy.org/D2908