fix: prevent panic in RedeemTokensForShares when too few tokens are redeemed

[sainoe]

Description

In RedeemTokensForShares(), this line establishes a 1:1 mapping between the redeemed token amount and shares.
However, this mapping is accurate only when a validator hasn't received any rewards and hasn't been slashed.

This PR changes the mapping by calculating shares as a fraction of the redeemed tokens
and the total share token supply, akin to the approach taken here.

Edit: This PR updates RedeemTokensForShares to return an error when the redemption amount is too small and truncates to 0 shares. That prevents a panic in SendCoins.

cc @sampocs

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[sampocs]

Apologies for the confusion, but can you elaborate on why this is inaccurate in the case of a validator having pending rewards or being slashed?

The LSM token is intended to map 1:1 with shares (it is minted based on shares, not tokens), so I was under the assumption that it would naturally capture rewards/slashing.

[sainoe]

The LSM token is intended to map 1:1 with shares (it is minted based on shares, not tokens), so I was under the assumption that it would naturally capture rewards/slashing.

You're 100% right. Sorry, I got it the wrong way. The simulation tests were failing due to these lines, which don't use a 1:1 mapping between LSM tokens and shares.