Use Specific Error Code for Index Not Found

Currently, the code 5000 is used for an index not found condition. It would be preferable to return this specific code:
https://github.com/couchbase/query/blob/master/errors/ds_couchbase.go#L106
given that this will occur if the indexes used by a prepared statement are deleted/unavailable. See the sequence below for an example.

cbq> prepare foo as select name, callsign from `travel-sample` where icao = "CAN"

cbq> drop index `travel-sample`.def_icao;

cbq> execute foo ;
{
"requestID": "2ddd790a-9416-465b-b201-344e79f3f45c",
"signature": null,
"results": [
],
"errors": [

{ "code": 5000, "msg": "index deleted or node hosting the index is down - cause: queryport.indexNotFound" }

],
"status": "errors",
"metrics":

{ "elapsedTime": "637.517µs", "executionTime": "562.341µs", "resultCount": 0, "resultSize": 0, "errorCount": 1 }

}