Return spray timeout error as JSON.

Otherwise, the UI cannot parse the response.  This is to fix #251.

### Before:
![cryptic_oti_upload_error_msg](https://cloud.githubusercontent.com/assets/960264/4511617/e730bab6-4b36-11e4-8c2e-a5a9900cdca9.png)

### After:
![image](https://cloud.githubusercontent.com/assets/960264/4511607/c9e734bc-4b36-11e4-971c-df9bb951b8b4.png)