Consider introducing a new error when shard size is not a multiple of 64

[Rootul P]

Context

reedsolomon/leopard8.go

Lines 136 to 138 in a6d2e3d

 if shardSize%64 != 0 { 
 	return ErrShardSize 
 } 

Problem

I observed the error shard sizes do not match when the shards in the test data I'm using actually do all match in length (they all have length 520 bytes).

Proposal

Introduce a new error that states something like shard size must be a multiple of 64 bytes