Silk error messages in log in debug mode

[lisad]

Mostly adding this to track the investigation we've done so far.

In DEBUG=True mode, Aaron & I both see

"Response to request with pk afe13179-c8a0-410b-92ea-dbed78468f67 has content type text/javascript but was unable to parse it"

in the logs. This is generated deep inside the silk performance profiling code. It doesn't seem to prevent silk from generating its statistics, so it's not a high priority to prevent/fix.