Check console exists before using

Summary:
Due to the fact that we now run tests in a worker, some challenges with minor errors in them
(normally non-fatal, just warnings) now wont work at all in Safari, as it reports console is undefined
This puts in a check that fixes that.

Test Plan:
- Put the mouse tracker code from live into dev mouse tracker, it should load in Safari
 (it has a minor error that causes console.warn to be triggered)
- Check that Chrome still warn's the error as before

Reviewers: john

Reviewed By: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D12302