Merge similar translation strings

we use "Page Not Found" in "inc/class-wpseo-options.php:1547"
and 
we use "Page not found" in "frontend/class-frontend.php:452"


If merged those two similar string, we will make it easier for the translations teams and decrease the .MO/.PO file size.
Merge pull request #1733 from ramiy/patch-4

Merge similar strings - Page not found