Pass ROSSRV class messages via ROS_ACOMMS

Using the roslib.messasge.get_service_class function, this allows ROSSRV class messages (e.g. ros_acomms_msgs/PingTranspondersRequest) to be passed via ROS_ACOMMS/ROS_IRIDIUM

Also adds some exception handling around codec setup and message parsing.

- id: 61
  message_codec: default
  subscribe_topic: !ros_param_substitute ${sub_cmd_ping; "send_iridium/cmd/transponder/ping"}
  publish_topic: !ros_param_substitute ${pub_cmd_ping; "from_iridium/cmd/transponder/ping"}
  ros_type: "ros_acomms_msgs/PingTranspondersRequest"
  default_dest: 221
  queue_order: fifo
  queue_maxsize: 1
  allow_fragmentation: false
  priority: 100
  fields:
    transponder_dest_mask:
      codec: fixed_len_array
      element_type: bool
      length: 4

    timeout_ms:
      codec: integer
      min_value: 1500
      max_value: 10000

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