Avoid standard command not found message when command-not-found is found

Squashed commit of the following:

commit c208bc30b7747b3743212483b3dd7e3f90819f49
Merge: 97bea94 2633372
Author: Konrad Borowski <glitchmr@myopera.com>
Date:   Fri Jul 19 09:56:12 2013 +0200

    Merge branch 'command-not-found' of git://github.com/GlitchMr/fish-shell into command-not-found

commit 26333721b9048333d1e7932505c221a31fd0e624
Author: Konrad Borowski <glitchmr@myopera.com>
Date:   Fri Jul 19 09:55:13 2013 +0200

    Fix command_not_found when not found

commit db34460bb51a4b1c1c456c2e535ae8d913d1071e
Author: Konrad Borowski <glitchmr@myopera.com>
Date:   Wed Jul 17 13:41:57 2013 +0200

    Avoid showing standard command not found message when possible

    In bash, command-not-found handler causes the standard messages to
    not appear. Because of events model in fish, it isn't really an
    option, so I moved the standard command not found message to
    fish function. This way, the messages aren't repeated, and the
    standard command not found message appears only when handler
    couldn't be found.