part 10: make output of `python -m unittest` example coherent with ex. 10B
Move "Exceptions" to part 09.

Renumber other slides accordingly.

  renamed:    ex11a.py -> ex09a.py
  renamed:    ex09a.py -> ex10a.py
  renamed:    ex09b.py -> ex10b.py
  renamed:    ex10a.py -> ex11a.py
  renamed:    ex10b.py -> ex11b.py
  renamed:    part11.tex -> part09.tex
  renamed:    part09.tex -> part10.tex
  renamed:    part10.tex -> part11.tex