This Fixes #548 : collapsing error traces

1> Created anchor tag to show/hide details in edit.html
2> Defined click function to show/hide details, error load in session_pane.js
3> Set display to none in ui_utils.js on click of show/hide details
4> Added style for show/hide details in edit.css
5> First three lines of the error are shown followed by the ‘Show Details’ link.
6> When ‘Show Details’ is clicked the error collapses to show the whole message followed by the ‘Hide Details’ message.
Screenshot attached after testing :
![show_details](https://cloud.githubusercontent.com/assets/6388509/5548707/7594e168-8b9e-11e4-8c84-876c6b1c3199.PNG)
![hide_details](https://cloud.githubusercontent.com/assets/6388509/5548710/7acdb722-8b9e-11e4-9a2d-866d1fe6bb43.PNG)

@gordonwoodhull 
Please review and let me know if this is fine.