Escape the message of an exception in debug_exceptions to avoid bad rendering

Sometimes, the exception message contains characters that may be interpreted as HTML, for instance :

``#<Hash:0x12345>`` is shown as # on chrome : 

![screen shot 2013-08-21 at 3 23 53 pm](https://f.cloud.github.com/assets/803765/1001293/630c0626-0a66-11e3-9f03-7cd566e848e9.png)

After the fix : 

![screen shot 2013-08-21 at 3 23 20 pm](https://f.cloud.github.com/assets/803765/1001294/6b263c78-0a66-11e3-8d79-ce410f37f42f.png)