[webauthn] Processing model for extensions is very underdefined

bzbarsky has just created a new issue for 
https://github.com/w3c/webauthn:

== Processing model for extensions is very underdefined ==
I'm not sure how well this can really be specified, but...

https://w3c.github.io/webauthn/#dom-webauthentication-makecredential 
step 7 says:

> If an error is encountered while processing an extension, skip that 
extension and do not produce any extension data for it.

It's not clear what that means.  Can "an error" include ES exceptions?
  I see no reason it can't, but then should those be caught or 
reported?

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/270 using your GitHub account