* spot/twaalgos/game.hh, spot/twaalgos/game.cc: Rename
solve_reachability_game() as solve_safety_game(), rewrite it (the old
implementation incorrectly marked dead states as winning for their
owner).
* tests/python/paritygame.ipynb: Rename as...
* tests/python/games.ipynb: ... this, and illustrate
solve_safety_game().
* tests/Makefile.am, NEWS, doc/org/tut.org: Adjust.
* tests/python/except.py: Add more tests.