Inspect both index and created keys for errored documents

For some reason, when migrating from an unaliased "metasearch",
documents come back with a "created" => key, rather than an "indexed"
key.

I have no idea why. But this stops the rake task from erroring and
allows the aliasing to complete successfully.
Merge pull request #289 from alphagov/allow-inspection-of-created-items

Inspect both index and created keys for errored documents