Extension language significantly clarified

https://github.com/w3c/webauthn/pull/389 has been updated to now consistently use the following terms for extension inputs, outputs, and processing:
[=client extension input=]
[=client extension processing=]
[=client extension output=]
[=authenticator extension input=]
[=authenticator extension processing=]
[=authenticator extension output=]

These are now distinct from these terms:
[=client data=]
[=authenticator data=]

The old wording often used the latter terms in extensions, where the more precise terms above were actually meant, resulting in very confusing exposition in places.  This is now corrected.

The extension error language was also updated, incorporating a suggestion from Vijay.

Hopefully we can decide to merge this in tomorrow.

                                                                -- Mike