In the function remove_shortcut, if the action failed, it used to
throw an 'implementation oriented' exception message:
> throws('try to delete non-leaf')
This is not a good message for the user who wants to mess with the
shortcuts.
The solution is to leave this 'implementation oriented' exception message
and add a try-catch bloc above that catches this message and throws
a new, frendliest, exception message.
I came up with this:
> throws('try to remove non-existing shortcut')
* notebook/static/base/js/keyboard.js: (remove_shortcut) add try-catch.
Signed-off-by: Nicolas Barray <nbarray@gmail.com>