Also, switch from returning false to throwing errors in most verify*()
functions, as well as in `await signatures[*].verified`, in order to be
able to show more informative error messages.
This also has the effect that we only throw on them when trying to use
the key, instead of when parsing it, and that we don't throw when the
authorized revocation key is specified in a separate direct-key
signature instead of a User ID self-signature (the spec only specifies
including it in a direct-key signature, so that means that we
effectively don't reject them anymore. This is because users that
wanted to use the key, could remove this separate signature, anyway.)
