Change a warning to distinguish between too few buttons and too many.

Before this change:

    $ xmodmap -e "pointer = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15"
    Warning: Only changing the first 15 of 10 buttons.
    $

After this change:

    $ xmodmap -e "pointer = 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15"
    Warning: Not changing 5 extra buttons beyond 10.
    $

Fixes: #2


Signed-off-by: Karl Fogel <kfogel@red-bean.com>