Check for a `primary` nav_menu location

[ehg]

In order for us to build tools that can manage a site/theme's menus, we need to know what the 'primary' location is for the theme - in a predictable manner.

Currently, the convention seems to be to register the 'primary' location first, and/or to have the name 'primary'.

Maybe we could check that a theme always registers a location named 'primary'?