[ros2doctor] Improve doctor_warn()

[claireyywang]

Updates in this PR:

• Decouple doctor_warn() and Result() to improve error/warning trace-back
  before, warning always trace back to the format.py file

  /home/claire/ros2_ws/build/ros2doctor/ros2doctor/api/format.py: 76: 
  UserWarning: Distribution foxy is not fully supported or tested. 
  To get more consistent features, download a stable version at https://index.ros.org/doc/ros2/Installation/
  

  now, warning points to the exact file where things go wrong

  /home/claire/ros2_ws/build/ros2doctor/ros2doctor/api/platform.py:76:
  UserWarning: Distribution foxy is not fully supported or tested. 
  To get more consistent features, download a stable version at https://index.ros.org/doc/ros2/Installation/
  

• Add doctor_error() in addition to doctor_warn(), to differentiate warning and error msgs.
• Use f-string for all print statements
• Shorten missing package names instead of printing them all out
  before

  /home/claire/ros2_ws/build/ros2doctor/ros2doctor/api/format.py: 76: 
  UserWarning: Cannot find required versions of packages: 
  rosbag2 rosbag2_compression zstd_vendor rviz_visual_testing_framework rviz2 rviz_default_plugins rviz_common ros2bag rosbag2_transport rosbag2_storage_default_plugins rosbag2_converter_default_plugins rosbag2_cpp rosbag2_storage yaml_cpp_vendor ros1_bridge interactive_markers common_interfaces visualization_msgs dummy_robot_bringup robot_state_publisher kdl_parser urdf turtlesim tracetools_launch topic_monitor tf2_tools geometry2 tf2_sensor_msgs tf2_kdl tf2_geometry_msgs tf2_eigen examples_tf2_py tf2_ros tf2_py tf2_msgs test_msgs sros2_cmake sros2 rqt_topic rqt_top rqt_srv rqt_shell rqt_service_caller rqt_reconfigure rqt_py_console rqt_publisher rqt_plot rqt_action rqt_msg rqt_console rqt rqt_py_common rqt_graph rqt_gui_py rqt_gui ros_testing ros2trace ros2topic ros2test ros2component ros2param ros2lifecycle ros2service ros2run ros2launch ros2pkg ros2node ros2multicast ros2interface ros2doctor ros2action ros2cli quality_of_service_demo_py quality_of_service_demo_cpp launch_testing_ros demo_nodes_cpp composition launch_ros examples_rclpy_minimal_subscriber examples_rclpy_minimal_service examples_rclpy_minimal_publisher examples_rclpy_minimal_client examples_rclpy_minimal_action_server
  

  now

  /home/claire/ros2_ws/build/ros2doctor/ros2doctor/api/package.py:118: 
  UserWarning: Cannot find required versions of packages: 
  rosbag2 rosbag2_compression zstd_vendor rviz_visual_testing_framework rviz2 [...] Use `ros2 doctor --report` to see full list.
  

• Keep error/warning logic consistent everywhere.

[sloretz]

Just a few comments; the rest LGTM

[claireyywang]

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