devices: proxy: print failed command on error
devices: proxy: print failed command on error

Add some context for debugging failures so it is possible to determine
which register read is failing.

BUG=None
TEST=./build_test.py

Change-Id: I6084971bc6dbd1f7b5d46e6c5d7ba017bb32edc6
Signed-off-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1893637
Reviewed-by: Zach Reizner <3374927928a53d04457674c546bd4de57c2044ec@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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