In the past, we've seen a couple of cache poisoning cases when we use remote cache/execution and we usually only get error messages like: java.io.IOException: Output download failed: Expected digest '...' does not match received digest '...'. It would be helpful if we can get the corrupted blob to debug with.

Steps:

• Add more details describing the error in Bazel. e.g. including file name, using a dedicated exception class.
• Don't delete (partial) downloaded outputs upon errors.
• Let Bazel CI catch the exception and upload blobs referenced in the error message to Buildkite.

#1174