Replace 'SEO not Available' with something more descriptive

[PatrickYoast]

Is your feature request related to a problem? Please describe.

At support we see a lot of tickets from user who say there SEO is not availabe. This is because the wording we use when no focus keyphrase is set. It seems to be confusing for users since they just installed Yoast SEO and now their SEO is not available.

Describe the solution you'd like

Change the wording. We could say something like "Focus Keyphrase not set" or something similair which indicates that an action is needed from the user.

[jonoalderson]

Let's change the wording to "N/A - Focus Keyphrase not set"

[omarreiss]

"N/A - Focus Keyphrase not set" doesn't translate well. Let's just do "Focus Keyphrase not set"