Reconciling with FedCM errors

[npm1]

At TPAC we had a breakout where we tried to dig into this but we didn't. @timcappalli mentioned that FedCM may want to reconcile with WebAuthn errors. I'm not familiar with the WebAuthn errors being exposed right now but I can explains what we are proposing in w3c-fedid/FedCM#498 to see what kind of coordination we would like to have. Filing this issue to perhaps get the interested folks in a call at some point to flesh this out? Note: surfacing new errors / discussions on the privacy implications of doing so is out of scope for this issue, I'm more interested in looking at the current status quo of each spec so that we can confidently merge the relevant FedCM PR.

[timcappalli]

Let's use the existing issue (#2096)