Return 404 instead of 400

Author: nielsbauman

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.rollover/10_basic.yml

@@ -187,7 +187,7 @@
 ---
 "Rolling over an unknown target should return 404":
   - do:
-      catch: bad_request
+      catch: missing
       indices.rollover:
         alias: "non_existent"
   - match: {error.reason: "rollover target [non_existent] does not exist"}

server/src/main/java/org/elasticsearch/action/admin/indices/rollover/MetadataRolloverService.java

@@ -11,6 +11,7 @@
 
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.elasticsearch.ResourceNotFoundException;
 import org.elasticsearch.action.admin.indices.create.CreateIndexClusterStateUpdateRequest;
 import org.elasticsearch.action.admin.indices.create.CreateIndexRequest;
 import org.elasticsearch.action.datastreams.autosharding.AutoShardingResult;
@@ -664,7 +665,7 @@ static void validate(
     ) {
         final IndexAbstraction indexAbstraction = project.getIndicesLookup().get(rolloverTarget);
         if (indexAbstraction == null) {
-            throw new IllegalArgumentException("rollover target [" + rolloverTarget + "] does not exist");
+            throw new ResourceNotFoundException("rollover target [" + rolloverTarget + "] does not exist");
         }
         if (VALID_ROLLOVER_TARGETS.contains(indexAbstraction.getType()) == false) {
             throw new IllegalArgumentException(