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