Remove unnecessary SFCState fields, errors rename

[thaarok]

Added documentation into SFCState comments, renamed few errors to be more descriptive.

Following fields, which are write-only for the SFC, only to be read from outside, removed:

• totalSlashedStake does not provide much useful information, it is even misleading - it includes only penalties subtracted from successful withdrawals. It does not contain slashed stake which was not withdrawn, or which even cannot be withdrawn because it is fully slashed.
• EpochSnapshot.totalBaseRewardWeight represents base (sum) for individual validators participation: sum( stake * uptimeRatio ^ 2 ) However without individual baseRewardWeights, the number seems worthless. (baseRewardWeights are used in the BaseReward calculation, but not stored anywhere)
• EpochSnapshot.totalTxRewardWeight is the same case, but for TxReward calculation: sum( originatedTxsFee * uptimeRatio )