Returns the current rate for exchanging funds from the sell account into the buy account: the amount of the
buy currency for one unit of the sell currency. Both accounts must be open and belong to the same client.The rate already includes the client's exchange fee. It is indicative and is not reserved; the rate applied
to an exchange is returned in the exchangeRate of the exchange transfer once it is executed.Errors:
404 Not Found - an account does not exist, belongs to another integrator or is not open;
422 Unprocessable Content - the accounts belong to different clients, or exchange between them is not
available.