buffet: Add RegistrationStatus::InvalidCredentials

If the device has been deleted from GCD then an oauth request will
return an error of invalid_grant. Make sure we handle this and set
the registration status to InvalidCredentials.

BUG=brillo:16
TEST=FEATURES=test emerge-${BOARD} buffet (I also tested this manually
by deregistering the device using
https://gcd.sandbox.google.com/clouddevices#) and
test_that <ipaddress> buffet_Registration buffet_BasicDBusAPI

Change-Id: I60aa68186d9c6f3a7ce812e4dddfda626ea2b42f
Reviewed-on: https://chromium-review.googlesource.com/254170
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Nathan Bullock <nathanbullock@google.com>
Tested-by: Nathan Bullock <nathanbullock@google.com>
diff --git a/buffet/registration_status.cc b/buffet/registration_status.cc
index 4fb1721..3b44ae3 100644
--- a/buffet/registration_status.cc
+++ b/buffet/registration_status.cc
@@ -18,6 +18,8 @@
       return "registering";
     case RegistrationStatus::kRegistered:
       return "registered";
+    case RegistrationStatus::kInvalidCredentials:
+      return "invalid_credentials";
   }
   return "unknown";
 }