fs_mgr_verity: allow verification error when the device is unlocked

On a A/B device, the image combination in VTS is:
  - system.img (userdebug): provided by Google (system as root)
  - vendor.img (user): provided by the OEM

The generic system.img provided by Google doesn't have /verity_key.
So verified boot will fail when it tries to verify the signature of
vendor.img. This CL allows signature verification error (including
no verity_key) when the device is unlocked.

This CL also changes the return value to FS_MGR_SETUP_VERITY_SKIPPED when
signature verification failed in logging mode. Otherwise, first stage
mount will fail because it assumes verity device initialization should be
successful when receiving FS_MGR_SETUP_VERITY_SUCCESS.

Bug: 63821912
Test: boot generic system.img on a A/B device
Change-Id: I33e5ef753913ae9f0c8b02c518ae94d4c8505611