Move authpolicy's ErrorType to a proto

Allows strongly typed last_auth_error proto member.

CQ-DEPEND=CL:488901

BUG=None
TEST=Compiles fine with platform2 changes.

Change-Id: Ia98944628b31bfd43a1b289dcf0bc5092db00927
Reviewed-on: https://chromium-review.googlesource.com/488881
Commit-Ready: Lutz Justen <ljusten@chromium.org>
Tested-by: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
2 files changed