Warn user if he/she is using x instead of * in expressions

Test Plan: Create an exercise with an answer of '<div class="solution" data-type="expression" data-same-form>5 * 5</div>' and try typing things like "5x5", "25x1", "5X5", etc.

Reviewers: alex, alpert

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4698