add ability to track number of errors and error types

[victoria-huang]

Addresses issue #20 - add number of errors (similar to numWrites) as well as error type (e.g. invalid tile/color/id) using the check in Validator.js

• Pushes all error types into a group's error type array (not sure if we should .uniq this, may be interesting to count how many times a group has placed an invalid tile/etc?)
• Add separate keys in Achievement's milestones hash for errors and errorTypes - should we count the number of error types/increase the achievement numbers for errors?
• Altered stringifyAchievements() to reflect changes
• Are there any other areas that we should be checking for errors?

Thanks!

[realAndrewCohn]

@victoria-huang are we good to delete this branch now that it's been merged?

[victoria-huang]

Yes, will delete!