If there are no errors in console, it's likely there were no errors, actually.
Another tab you can inspect is the network, to see all the requests to XData server and check if any of them failed with some error response.
I was using the wizard for the XData Web Application as a starting point. The first form I was trying had many fields and the top of the form was offscreen.. The error was at the top of the form. And I hadnt realised it was there.
If pressed Save again, the form just closed, implying that everything was fine. Which is why I was confused. I will add some code to disable the Save button on error.
Also, I will make the error pop up or find a way to scroll the form up.