Short description:

Binding a combination such as Ctrl-J to an action is not flagged as an error,
even though there are existing multi-key shortcuts that start with Ctrl-J.

Long description:

The UI currently warns the user when attempting to assign a key combination that
is currently in use, but only when the conflict is with a simple key binding. 
The user gets no warning when it is a multi-key binding.  This is very
frustrating for the user since it appears that nothing happens when they press
the selected key.

Reproduction:
Using the netbeans profile, add a new keybinding to the insertion-point-up
action.  Click OK and edit an existing file.  Press Ctrl-J.  Nothing seems to
happen.  You have to be really alert to see the "Ctrl+J" on the left side of the
status bar.

The problem is that the netbeans profile has an existing keybinding for Ctrl+J,
D to macro-debug-var.  This makes it even harder for the user to find, since
macro keybindings are set in a different panel.

The best fix for this would be to show the user all the keys that conflict with
the chosen binding and allow the user to override all of them if desired.

But at the very least there should be a warning as to what the situation is.