We have recently released an improvement for custom cards error communication. It's now possible to see when a custom card has an error and the error code for it. Moreover, you can expand the error message for further investigation and troubleshooting.
When custom card has an error there's an exclamation mark visible in the sidebar
Clicking the highlighted custom card will expose more information about it which can be used for further troubleshooting
Error details show a server response that can be copied and shared with an admin or support.