[Debug] enhanced error messages for uncaught exceptions

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

Before:

![capture du 2014-06-02 10 56 42](https://cloud.githubusercontent.com/assets/243674/3145281/3f8c4d24-ea34-11e3-948c-78898fb79a75.png)

After:

![capture du 2014-06-02 10 55 57](https://cloud.githubusercontent.com/assets/243674/3145283/44167a86-ea34-11e3-9786-f77f2f001638.png)