testing 'beginner' difficulty
    loading from file mines_beginner.txt
Traceback (most recent call last):
  File "./example.py", line 502, in <module>
    main()
  File "./example.py", line 489, in main
    expert="mines_expert.txt")
  File "/mnt/tera1/devel/src/mlppzl/minesweeper.py", line 441, in testSolver
    grids = loadMinefieldsFromFile(params[testName])
  File "/mnt/tera1/devel/src/mlppzl/minesweeper.py", line 388, in loadMinefieldsFromFile
    return [Minefield.LoadMinesFromString(s) for s in gridStrings]
  File "/mnt/tera1/devel/src/mlppzl/minesweeper.py", line 336, in LoadMinesFromString
    assert len(lines)-1 == nrows