Fixes indexing error response handling

With an indexing error dopplr tries to access a field that does not exist.

    curl -XPOST http://localhost:8983/solr/core/update/json -H "Accept: application/json" -H "Content-Type: application/json" -d '{"asdf": 123}'

    {"responseHeader":{"status":400,"QTime":0},"error":{"msg":"Unknown command: asdf [9]","code":400}}