Yelp string change (again)

[Don Scorgie <Don Scorgie org>]

Hi,

In yelp-window.c I've added a new string "Phrase not found".  This is
shown in the search bar in addition to the beeping / flashing that
happens when a find term isn't found.  This provides better feedback to
the user rather than just beeping rudely at them.

Thanks
Don