guest_memory: implement error trait
guest_memory: implement error trait

This implements the error trait so we can get more useful error
information back on failures.

BUG=chromium:797868
TEST=./build_test passes on all architectures
TEST=crosvm runs on caroline

Change-Id: Ia131b1eb14d24709c10f2ba752a962e822134a42
Reviewed-on: https://chromium-review.googlesource.com/938312
Commit-Ready: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Tested-by: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>
Reviewed-by: Sonny Rao <d867fc987a366be56684d1145622524a3ba1efbf@chromium.org>

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