WTF~~
Why this grid

[[0, 0, 0, 4],
[0, 4, 2, 0],
[0, 0, 0, 0],
[0, 3, 0, 1]]

expects 'unsolvable'
instead of

[[3, 2, 1, 4],
[1, 4, 2, 3],
[4, 1, 3, 2],
[2, 3, 4, 1]]

1. It is unclear whether the subsquares should be validated or they are not considered as it was initially.
   Please add it to the description.
   
2. There are still some issues with the random tests:

Given: [ [ 0, 0, 2, 0 ], [ 0, 0, 0, 3 ], [ 0, 0, 0, 4 ], [ 0, 0, 3, 0 ] ]
Expected: 'This sudoku is unsolvable!', instead got: [[3, 4, 2, 1], [1, 2, 4, 3], [2, 3, 1, 4], [4, 1, 3, 2]]

Given: [ [ 2, 4, 3, 1 ], [ 0, 0, 0, 0 ], [ 3, 1, 0, 4 ], [ 4, 2, 0, 0 ] ]
Expected: 'This sudoku is unsolvable!', instead got: [[2, 4, 3, 1], [1, 3, 4, 2], [3, 1, 2, 4], [4, 2, 1, 3]]

Is there a problem with the etalon solution?

P.S. Anyway, the kata is better now :)