Show block on all 404 / 403 pages only via context
A better way to do this is create a panel page for 403 and 404 page. Add whatever you want to in the panes.

Set the 403 and 404 page path under :

Admin &gt; Configuration &gt; System &gt; Site Information


There is a section called ERROR PAGES. Add both the path there.

Edit : 

For context use Context Error module.