[futility] Open gscvd in R/O mode when verifying

Otherwise, it will mutate the file under your feet!
This also adds some clarifying messages, including a message if
verification failed due to incorrect signing instead of silent output.

TEST=`futility gscvd <file>` does not change sha256sum of file
Signed-off-by: Alyssa Haroldsen <kupiakos@google.com>

Change-Id: Ic793ad47c0160e3cedb7e2b7cc842a3f4380749d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/3584690
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Commit-Queue: Alyssa Haroldsen <kupiakos@google.com>
Tested-by: Alyssa Haroldsen <kupiakos@google.com>
2 files changed