sequencer, mempool: make mempool insertion errors distinct abci errors

[Lilyjjo]

The mempool will reject transactions from the mempool when:

• The nonce is too low (InsertionError::NonceTooLow)
• There is already a transaction with that nonce for the account in the mempool (InsertionError::AlreadyPresent)
• The parked account limit is hit (InsertionError::AccountSizeLimit)

We return these errors as AbciErrorCodes. Nonce too low has the code AbciErrorCode::INVALID_NONCE while the other two cases share TRANSACTION_INSERTION_FAILED. It might be nice to have distinct error codes for the other two cases as well.

┆Issue Number: ENG-900

[Lilyjjo]

Implemented in #1515