add back LastGoodSquare to errors

[liamsi]

Revert removing the LastGoodSquare from the byzantine row/col errors (in #24).

copy-pasting @musalbas' clarification here:

It's so that fraud proofs can be generated for invalid codes, using the eds as much as it can be reconstructed.
It's not OK to remove LastGoodSquare

@adlerjohn had some ideas on how to add it without doing a deep-copy:

Basically, instead of corrupting the EDS here https://github.com/lazyledger/rsmt2d/blob/6610ba486a29d90dd0faa51f539da1415777a8f2/extendeddatacrossword.go?ts=4#L152-L158 then doing the check https://github.com/lazyledger/rsmt2d/blob/6610ba486a29d90dd0faa51f539da1415777a8f2/extendeddatacrossword.go?ts=4#L161-L170, do the check first then set the EDS in a way that's guaranteed correct. The check would have to compute a root based on existing leaves and a new set of repaired leaves, but that's just copy-pasting this logic https://github.com/lazyledger/rsmt2d/blob/6610ba486a29d90dd0faa51f539da1415777a8f2/extendeddatacrossword.go?ts=4#L152-L158.

But we can also just add it back as before and do the optimization later.

More important than above mentioned optimzations are test-cases (additional to go-doc comments) that document the purpose of the LastGoodSquare fields, for example by generating above mentioned fraud proofs (also related to #12).