Also improve usefulness of some blockstore runtime logs for debugging

Context:

Sometimes when trying to load an XBlock's XML file from Amazon S3, AWS will return a 4xx or 5xx response along with error XML like:

    <Error><Code>NoSuchKey</Code><Message>The specified key does not exist.</Message><Key>foo/bar</Key>...</Error>

A bug in the get_bundle_file_data_with_cache method would cause this XML to be returned to the runtime anyways, as if it were the expected OLX. This would then (obviously) lead to strange parsing bugs, e.g. when trying to interpret <Code> as an <xblock-include>.

This fixes the bug and improves the logging, both to make this sort of issue easier to debug in the future and to return whatever detailed error code S3 provides (or Blockstore, if S3 is not being used).