Descriptive error messages for julia exceptions

This patch brings more descriptive error messages when julia code throws an exception. Example output in IPython notebook:

![IPython notebook julia error example image](https://cloud.githubusercontent.com/assets/313307/4098016/9d1ecd84-2ff5-11e4-8506-d24155d1bef9.png)
Would be simpler to just do `sprint(showerror, e, catch_backtrace())`