Add a public error code to DexFileLoader::OpenAll.

Some of the APKs in the build system don't contain any dex file (e.g.
they are only there to test that PackageManager can install them).
Users of this library (e.g. veridex) can now verify the error condition
to determine whether they should just skip the file or propagate the
error.

Test: m test-art-host-gtest; ran veridex on an APK without classes.dex
Bug: 110073830
Bug: 78924201
Change-Id: I65b6a5aa8abb404a77b90352dd066fb0dff36955