Catch shard failure and throw reasonable exception when doing a fair query against a non-existing mtable

[tsuehr]

Executing
POST /test/_search
{"query" : {
"match" : {
"body" : "hello"
}
},
"rescore" : {
"fair_rescorer" : {
"protected_key" : "gender",
"protected_value" : "f",
"significance_level" : 0.1,
"min_proportion_protected" : 0.8
}
}
}

without doing before

POST /_fs/_mtable/0.8/0.1/10

will lead to All shards failed error within elasticsearch.
Some error message like "mtable does not exist" would be better