Improved error checking for fsverity_init/odsign.

When attempting to load a non-existent cert I got:
  06-10 12:48:11.939   662   662 E fsverity_init: Failed to add key: Invalid argument
  06-10 12:48:11.940   662   662 E fsverity_init: Failed to load key from stdin
  06-10 12:48:11.941   648   648 I odsign  : Added CompOs key to fs-verity keyring
Which looks like everything worked when nothing did.

Added more error checks on both sides.

Test: Presubmits
Test: Manual
Change-Id: Ib2b17ce75e58dafb0ad6905106e35b11b55e91d0
2 files changed