Test flake reporting excessive responses

I just saw this strange error/flake on circleci:

```
======================================================================
ERROR [2.608s]: test_missing_data_throws_error (test_browser.browser)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/root/project/test/test_browser.py", line 830, in test_missing_data_throws_error
test()
File "/root/project/test/test_browser.py", line 823, in test
self.run_browser('test.html', '/report_result?1')
File "/root/project/test/common.py", line 2438, in run_browser
self.assert_out_queue_empty('this test')
File "/root/project/test/common.py", line 2376, in assert_out_queue_empty
raise Exception('excessive responses from %s: %s' % (who, '\n'.join(responses)))
TypeError: sequence item 1: expected str instance, NoneType found
```