Make execTransactionIfApproved handling of failed transactions in team addition consistent with personal edition

[Georgi87]

In the personal edition the nonce is increased even in case the transaction fails. We should have the same behavior for team edition: mark a transaction as executed even in case it fails and signal a failing transaction with an event. This prevents that transactions, which might not work at the time of transaction submission to remain forever in pending state and get lost, which is a potential security issue. Also execTransactionIfApproved should then return the success status of the transaction.

[rmeissner]

I implemented this on https://github.com/gnosis/safe-contracts/tree/feature/improve_team_edition

I was asking myself if it would make more sense to provide a clearApproval. Mainly because if by any change the required gas changes and the transaction fails because of not enough gas, all confirmations have to be gathered again.

[Georgi87]

Who should be allowed to call clearApproval? This would require another multisig tx?

[rmeissner]

@Georgi87 it would be more that each owner can clear his own approval (actually it shouldn't be required to be an owner to clear your approval, so that you can remove your approval even after you are not an owner anymore)

[rmeissner]

Close because of #53