Add signal for container startup failures for VMs

This adds the constant for the signal name and amends the comment for
the ContainerStartedSignal protobuf to indicate it is for general
container startup messaging.

BUG=None
TEST=Builds

Change-Id: I9e6a5082bda29a9cc6d94ec4aa49c499a7ee3b71
Reviewed-on: https://chromium-review.googlesource.com/1011192
Commit-Ready: Jeffrey Kardatzke <jkardatzke@google.com>
Tested-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
2 files changed