OrderedMap: report error if no top map-field exists in sheet message

[wenchy]

For example, ItemConf has no map field, but worksheet option ordered_map has been set to true:

message ItemConf {
  option (tableau.worksheet) = {name:"Item" ordered_map:true};

  int64 max_recent_cnt = 1 [(tableau.field) = {name:"MaxRecentCnt"}];
  int64 max_recent_limit = 2 [(tableau.field) = {name:"MaxRecentLimit"}];
}