Moving directly to copy_execution_state step in ILM causes NullPointerException

[dakrone]

When moving directly to the copy_execution_state step using move-to-step, a
NullPointerException shows up as the cause of the error step. Rather than
running into this, we should guard against this and return a more helpful
exception to the user to let them know how to fix it.

Reproduction:

PUT _ilm/policy/my_lifecycle3
{
  "policy": {
    "phases": {
      "hot": {
        "min_age": "20m",
        "actions": {
          "rollover": {
            "max_age": "5s"
          }
        }
      },
      "warm": {
        "min_age": "5s",
        "actions": {
          "forcemerge": {
            "max_num_segments": 1
          },
          "shrink": {
            "number_of_shards": 1
          },
          "allocate": {
            "include": {
              "type": ""
            },
            "exclude": {},
            "require": {}
          }
        }
      },
      "cold": {
        "min_age": "10s",
        "actions": {
          "allocate": {
            "number_of_replicas": 2,
            "include": {
              "type": "cold"
            },
            "exclude": {},
            "require": {}
          }
        }
      },
      "delete": {
        "min_age": "2m",
        "actions": {
          "delete": {}
        }
      }
    }
  }
}

PUT _template/my_template
{
  "index_patterns": ["test-*"],
  "settings": {
    "number_of_shards": 2,
    "number_of_replicas": 1,
    "index.lifecycle.name": "my_lifecycle3",
    "index.lifecycle.rollover_alias": "test-alias",
    "index.routing.allocation.include.type": "hot"
  }
}

PUT test-000001
{
  "aliases": {
    "test-alias":{
      "is_write_index": true
    }
  }
}

POST /_ilm/move/test-000001
{
  "current_step": {
    "phase": "new",
    "action": "complete",
    "name": "complete"
  },
  "next_step": {
    "phase": "warm",
    "action": "shrink",
    "name": "copy_execution_state"
  }
}

GET /*/_ilm/explain

Produces the following JSON:

{
  "indices" : {
    "test-000001" : {
      "index" : "test-000001",
      "managed" : true,
      "policy" : "my_lifecycle3",
      "lifecycle_date_millis" : 1542235008615,
      "phase" : "warm",
      "phase_time_millis" : 1542235010235,
      "action" : "shrink",
      "action_time_millis" : 1542235010235,
      "step" : "ERROR",
      "step_time_millis" : 1542235010250,
      "failed_step" : "copy_execution_state",
      "step_info" : {
        "type" : "null_pointer_exception",
        "reason" : null,
        "stack_trace" : "java.lang.NullPointerException\n\tat org.elasticsearch.cluster.metadata.IndexMetaData$Builder.<init>(IndexMetaData.java:858)\n\tat org.elasticsearch.cluster.metadata.IndexMetaData.builder(IndexMetaData.java:829)\n\tat org.elasticsearch.xpack.core.indexlifecycle.CopyExecutionStateStep.performAction(CopyExecutionStateStep.java:67)\n\tat org.elasticsearch.xpack.indexlifecycle.ExecuteStepsUpdateTask.execute(ExecuteStepsUpdateTask.java:96)\n\tat org.elasticsearch.cluster.ClusterStateUpdateTask.execute(ClusterStateUpdateTask.java:47)\n\tat org.elasticsearch.cluster.service.MasterService.executeTasks(MasterService.java:642)\n\tat org.elasticsearch.cluster.service.MasterService.calculateTaskOutputs(MasterService.java:271)\n\tat org.elasticsearch.cluster.service.MasterService.runTasks(MasterService.java:201)\n\tat org.elasticsearch.cluster.service.MasterService$Batcher.run(MasterService.java:136)\n\tat org.elasticsearch.cluster.service.TaskBatcher.runIfNotProcessed(TaskBatcher.java:150)\n\tat org.elasticsearch.cluster.service.TaskBatcher$BatchedTask.run(TaskBatcher.java:188)\n\tat org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:627)\n\tat org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:244)\n\tat org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:207)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)\n\tat java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)\n\tat java.base/java.lang.Thread.run(Thread.java:834)\n"
      },
      "phase_execution" : {
        "policy" : "my_lifecycle3",
        "phase_definition" : {
          "min_age" : "5s",
          "actions" : {
            "allocate" : {
              "include" : {
                "type" : ""
              },
              "exclude" : { },
              "require" : { }
            },
            "shrink" : {
              "number_of_shards" : 1
            },
            "forcemerge" : {
              "max_num_segments" : 1
            }
          }
        },
        "version" : 1,
        "modified_date_in_millis" : 1542235004651
      }
    }
  }
}

[elasticmachine]

Pinging @elastic/es-core-infra