framework: change "check_for_man" return value upon failure
    
    check_for_man is designed to be used by plugins, which might "save"
    the return value in "args" using "report" (viz. checkall_boolean).
    In that case None is changed to string "None", causing hard to find
    bugs when recognising non-existent man page.
    
    Signed-off-by: Vit Mojzis <vmojzis@redhat.com>