location-entry: Crash nicer if paste menuitem can't be found

This is really unfortunate design, but I don't see any better way to
implement the code, so we'll have to crash if the translation is wrong.

Let's give a nice error message at least, and add a warning for the
translators, since this is quite a trap otherwise.

https://bugzilla.gnome.org/show_bug.cgi?id=772994