When checking for the existence of a document in the ILM/CCR integration
tests, `assertDocumentExists` makes an HTTP request and checks the
response code. However, if the repsonse code is not successful, the call
will throw a `ResponseException`. `assertDocumentExists` is often called
inside an `assertBusy`, and wrapping the `ResponseException` in an
`AssertionError` will allow the `assertBusy` to retry.
In particular, this fixes an issue with `testCCRUnfollowDuringSnapshot`
where the index in question may still be closed when the document is
requested.
c353ad71fe