Adding error for when overlayfs is enabled in OTA

when running adb remount, overlayfs is enabled on the device interfering
with OTA being able to run. Currently we don't have a good debug message
and users aren't able to realize why the OTA fails. Adding this log will
make it clear this behavior is intended, and we need verity-enabled for
OTA to work

Test: adb remount, update_device.py
Bug: 286889613
Change-Id: Ibcd27911afe6f02ddf0c38ad40904a7e17735b31
diff --git a/common/error_code.h b/common/error_code.h
index a889888..7924579 100644
--- a/common/error_code.h
+++ b/common/error_code.h
@@ -87,6 +87,7 @@
   kDeviceCorrupted = 61,
   kPackageExcludedFromUpdate = 62,
   kPostInstallMountError = 63,
+  kOverlayfsenabledError = 64,
 
   // VERY IMPORTANT! When adding new error codes:
   //