Crude error checking for logbook widgets

This 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.
Oh - also fixes a crash with trying to load nonexistent 14_bold fonts.
Fortunately [BoldLabel] was unused until now, so it went unnoticed.
Use BoldLabel for tutorial task summary lines

Now that it works, why not use it?
Note that this changes translatable strings in that I got rid of the
(now) unnecessary capitalization of TASK.
Merge pull request #2182 from unknown-horizons/logbook-errors

Crude error checking for logbook widgets