Simplify errors

[stechu]

Get rid of stand alone LedgerInternalError

[bhgomes]

What I meant for structuring the errors was something like the following:

trait TransferLedger {
    /// State Update Error
    type UpdateError;

    fn update_public_balances(...) -> Result<(), Self::UpdateError>;
}

where this error is core::convert::Infallible for the mock ledger and for substrate it can be some other error.

Also, please remove the dead code introduced by the previous PR, namely the AccountBalance enum and the LedgerInternalError struct.

[bhgomes]

We basically want the ledger to state whether updating the public balances can fail or not. In the case of the mock ledger, it cannot so the error type is Infallible but for substrate, we may have some more complex requirements. Eventually, we want to revisit the error states of the ledgers since they assume no errors during other state updates but we may want to make this explicit. Currently, the API is a bit asymmetric because only update_public_balances gets an error condition, but this is fine for now.