Don't display an error when the theme supports widgets but not menus

[nacin]

Currently we display an annoying red bar when the theme doesn't support menus, but does support widgets.

Let's only have a bar (and make it yellow) before we've done anything. Once we have a menu, we can re-use the theme locations box to say "The current theme does not natively support menus, but you can use the "Custom Menu" widget to add any menus you create here to the theme's sidebar."

Note that this would not be a new string.

[nacin]

I'm also now realizing that calling add_theme_support('nav-menus') and register_nav_menus() are two different things and if only the former is done (as was the original instance) we may have some peculiar results.

[nacin]

(In [15191]) Don't show an error when the theme supports widgets but not menus. see #13825.

[nacin]

(In [15193]) Switch to current_theme_supports(menus) internally. Adding theme support for 'nav-menus' is deprecated in favor of register_nav_menu(s). fixes #13825.