Better errors on missing composite disk components.
Better errors on missing composite disk components.

When there is an error opening one of the composite disk components now,
it gives the message `failed to open component file: "No such file or
directory (os error 2)"` without specifying the file path it tried to
use. Exposing the file path will make it faster to act on errors, rather
than trying to examine the composite disk file for paths.

TEST=n/a
BUG=b:150150052

Change-Id: I9341b330e7e6dcd517d5bfb5262b1657a2da46fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2072738
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Cody Schuffelen <7618f12f8c1ef40a3c8bf8909fc72185c1ec8ce8@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm