Reword assertion message

Summary: Devin from Dartmouth suggested, I also find it easier to read.

Test Plan:
- Put this in a program:
```
Program.assertEqual(2, 4);
Program.assertEqual(3, 5);
```
- In Chrome, see messages on each line
- In Safari, see both messages on first line

Reviewers: brian

Reviewed By: brian

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D12640
Fix infinite loop with getImage

Summary: The live-editor merge missed this. See http://phabricator.khanacademy.org/D11134

Test Plan: - See http://phabricator.khanacademy.org/D11134

Reviewers: john

Reviewed By: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D12639
Don't show draw canvas upon load

Summary:
I'm not sure what we were previously doing (pre-live-editor),
but we don't want to show the draw canvas upon load.
Instead, we should only show it when draw commands start (drawStarted)
and then hide it when draw commands end (drawEnded).
This fixes the issue with users unable to interact (T3046).

Test Plan:
- Watch Modifying arrays, confirm you can press to see balloons at 00:37
- Watch Intro to Drawing, confirm you can see the drawings show up at 5:00

Reviewers: john

Reviewed By: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D12638
Fix merge with master.