Use more descriptive errors #212

WebAuthn is likely going to move toward a WebAuthn-specific error rather than trying to fit in existing DOMException type errors. That makes sense to me, and we need to support more than just WebAuthn errors, so we should return our own errors that can be mapped by clients to WebAuthn errors.

We should be careful to preserve the privacy properties that the current errors exhibit: an unprivileged caller should not be able to learn that a user has or does not have credentials without their consent.