Change viewport error messages in preparation for use with @viewport rule

The error messages will soon be used for both @viewport and viewport related
meta tags, so make the text less meta tag specific.

Review URL: https://chromiumcodereview.appspot.com/22908011

git-svn-id: svn://svn.chromium.org/blink/trunk@156424 bbb929c8-8fbe-4397-9dbb-9b2b20218538