Skip operator in do_slash_operators if it is not found

[NingLin-P]

The fraud proof will slash and remove the malicious operator immediately when the current epoch is ended while the same malicious operator may still get slashed due to invalid bundle later when the ER is confirmed. But at that time, since the malicious operator is already removed, the do_slash_operators will return UnknownOperator and cause any submit_bundle call fail during execution thus the domain chain is stopped forever.

This PR fixes the issue by simply skipping the operator in do_slash_operators if it is not found, this is okay since the operator has already slashed previously.

This also makes me think about another broader issue that there are staking related operations (i.e. slash/reward/withdraw/epoch transition) performed during the submit_bundle call, but the result of these operations is mostly unrelated to the content of the submit_bundle call, which means if the operation is failed it will fail for any bundle it is unrecoverable, so we must avoid any failure! I took a look at all the different operations and it seems to be fine now, but I could miss something, would be great to have @vedhavyas take a look.

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[vedhavyas]

Make sense to me