Feature Request: Don't fail YNAB exporter on unmapped accounts

[whatuserever]

Is your feature request related to a problem? Please describe.
I want to use Caspion to export transactions into YNAB, but I have multiple accounts on the same financial providers and I don't want to export all of them.
This is impossible at the moment. If your YNAB config has unmapped accounts, the YNAB exporter hangs and logs the following:

Unhandled account number <accountNumber>

Describe the solution you'd like
Simply ignore transactions from accounts that have no YNAB mapping. Emit a message in the logger indicating which accounts were skipped.

Describe alternatives you've considered
I see 2 options:

1. Always ignore unmapped accounts
2. Add an option in the config to allow ignoring unmapped accounts. If it's enabled - skip. Otherwise - error.

Additional context
The root cause is the throw statement here:

caspion/packages/main/src/backend/export/outputVendors/ynab/ynab.ts

Lines 136 to 142 in 3e7c007

	function getYnabAccountIdByAccountNumberFromTransaction(transactionAccountNumber: string): string {
	const ynabAccountId = ynabConfig!.options.accountNumbersToYnabAccountIds[transactionAccountNumber];
	if (!ynabAccountId) {
	throw new Error(`Unhandled account number ${transactionAccountNumber}`);
	}
	return ynabAccountId;
	}

I think the proper solution would be to refactor the YNAB exporter such that it exports transactions by account. This way the skip can be done before processing the transactions, and we'll have a single log line per skipped account instead of a line per transaction from skipped accounts.

Something along the lines of:

const createTransactions: ExportTransactionsFunction = async ({ transactionsToCreate, startDate }, eventPublisher) => {
  // ...
  const transactionsByAccount = // mapping from accountNumber to list of transactions from that account
  const results = transactionsByAccount.forEach(createTransactionsByAccount(accountNumber, accountTransactions))
  return sum(results)
}

function createTransactionsByAccount(accountNumber, accountTransactions) {
  if (unmapped(accountNumber)) {
    emit("skipping message") // or throw, depending on the decided approach
    return
  }
  // process transactions as before
}

I'm willing to work on this, but let me know what you think first :)