Cluster verify: factor out error codes and functions

We move all error code definitions, plus the _Error and _ErrorIf helpers,
to a private _VerifyErrors mix-in class that can be later shared by the new
two cluster verify LUs.

(_Error and _ErrorIf code was moved around verbatim, except to disable
"_VerifyError class does not have 'op' or '_feedback_fn' members" errors
from pylint.)

Signed-off-by: Adeodato Simo <dato@google.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>