prevent xss in the error page

One simple alteration to prevent xss attacks. 
Now, the page use one jinja's filter to escaping the exception message. 

![before](https://cloud.githubusercontent.com/assets/353515/4815246/4589f3aa-5ed5-11e4-9e35-6f2f3cf3065e.png)
![after](https://cloud.githubusercontent.com/assets/353515/4815248/4883f84e-5ed5-11e4-8fbf-877228abe84e.png)