Crude error checking for logbook widgets 

Fixes a crash with trying to load nonexistent `14_bold` fonts, and some more crashes with old image paths in scenario savegames (by not displaying these images).
Fortunately `BoldLabel` was unused until now, so that one went unnoticed.

of course this PR makes awful code more awful, but looking at our logbook i'm pretty sure nobody will notice.
Also can somebody please go and rewrite all of this... from scratch.