diskcache: ignore files that go missing during filepath.Walk

Created by: keegancsmith

For example this can happen when diskcache renames a temporary file. We shouldn't bail out early, but rather lets work on best effort information.

Test Plan: this is tough to reproduce, so instead I am relying on tests to catch any possible regressions.

Part of https://github.com/sourcegraph/sourcegraph/issues/34828