Provide error recap/summary at the end of a large build in tl

[rainersigwald]

In a very large build, errors may have scrolled well offscreen before the end of the build, leaving a terminal window with a sea of green "project succeeded" messages followed by an unhelpful "build failed with 8 errors".

We decided not to repeat every error because of noise, but we could consider a heuristic based on terminal height that could emit an error recap if errors were emitted but are now offscreen.

(I could swear we had this conversation several times but I can't find a bug tracking it)