Add error handling to GetIndicesPointingToAlias. /wrongname/_aliases used to return an empty array
(e.g. in ES 0.20) for a non-existing alias, but in more recent ES versions (0.90) it returns
{"error":"IndexMissingException[[wrongname] missing]","status":404}
So we need to add explicit error handling here, otherwise GetIndicesPointingToAlias returns two
"indices" called "error" and "status".