Internal: dangling indices import ignores aliases

Dangling indices are indexes found on disk which are not part of the cluster state. By default, we don't delete them but rather import them into the cluster state in order to not accidentally delete data and also allow for the ease of copying index data folders from one cluster to another. Currently, the import logic doesn't check for existing aliases of the same name as the imported dangling index, causing both an index and an alias with the same name.

 This commit add a protection against this. Note that the index is still kept as dangling and is only deleted from disk after `gateway.local.dangling_timeout` has passed (2 hours).

 We also change the log message indicating deletion of dangling indices to a `WARN` level.
@bleskes has the concern that this state is not defined in our system (since we protect on it from the "outside"), and the implications of having this clash is not evident. I was leaning towards living with the clash, but since its not defined, this change might be good enough for now.