Replace submit_transaction endpoint with less hopeless one :D

[dpc]

        api_endpoint! {
            SUBMIT_TRANSACTION_ENDPOINT,
            async |fedimint: &ConsensusApi, _context, transaction: SerdeTransaction| -> SerdeModuleEncoding<Result<TransactionId, TransactionError>> {
                let transaction = transaction
                    .try_into_inner(&fedimint.modules.decoder_registry())
                    .map_err(|e| ApiError::bad_request(e.to_string()))?;

                // we return an inner error if and only if the submitted transaction is
                // invalid and will be rejected if we were to submit it to consensus
                Ok((&fedimint.submit_transaction(transaction).await).into())
            }
        },

SerdeModuleEncoding<Result<TransactionId, TransactionError>>. That's terrible. Basically we encode the result using consensus encoding, making it impossible to evolve.

The top-level error is:

pub struct ApiError {
    pub code: i32,
    pub message: String,
}

so we should just impl From<TransactionError> for ApiError, and convert every case to a specific code, and throw the to_string() of the error into a message.

This will break the backward compat, so we should just make a new call for it.

Generally the way we handle errors in api is terrible, AFAICT.

BTW. If ApiError::code is the jsonrpc error code, then we should use better values, as per https://www.jsonrpc.org/specification , not http codes (see impls for ApiError):

The remainder of the space is available for application defined errors.