Google Sheets: a cell calls an API for a changing currency price, but sometimes returns an error. How do I keep the last non-error price?

I'm using importJSON to grab a number that represents the current price of a currency. Sometimes, the API isn't available and returns an error message. Is there a way for me to record the last non-error value in a different cell? A non-script way would be preferable, but I'll take anything at this point.