grains error message should be updated

[BNAndras]

Batch only works up to square 31 per the test suite comments. The expected error message for a square below 1 or above 64 is "square must be between 1 and 64", but no square above 31 would work. A more appropriate error message might be "square must be between 1 and 31".

Also we may want an instructions.append.md to highlight that Batch can only go up to square 31 so the tests have been tweaked around that. Then the students are aware of this limitation going in so there'll hopefully be less frustration.