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".