Wrap python2.7 error message with user-friendly error message

[rnjudge]

Describe the Feature
If you run Tern in an environment that defaults to Python2.7 (even with python3 installed) you get the following output:

Traceback (most recent call last):
  File "/usr/local/bin/tern", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python2.7/site-packages/tern/__main__.py", line 128, in main
    do_main(args)
  File "/usr/local/lib/python2.7/site-packages/tern/__main__.py", line 73, in do_main
    report.execute_docker_image(args)
  File "/usr/local/lib/python2.7/site-packages/tern/report/report.py", line 438, in execute_docker_image
    full_image = load_full_image(args.docker_image)
  File "/usr/local/lib/python2.7/site-packages/tern/report/report.py", line 140, in load_full_image
    test_image = DockerImage(image_tag_string)
  File "/usr/local/lib/python2.7/site-packages/tern/classes/docker_image.py", line 30, in __init__
    super().__init__(image_id)
TypeError: super() takes at least 1 argument (0 given)

I propose wrapping the raw output with a more user-friendly error message. Something to the effect of "Error running Tern. Please check that python3 is configured as default."

Use Cases
This would be helpful to new users who do think they have python3 installed on their system but do not have it configured as default.

Implementation Changes
The error is being hit in tern/classes/docker_image.py. Might consider using the traceback library: https://docs.python.org/3/library/traceback.html

Another option would be for Tern to check for dependencies somehow before running.

[AyahRamahi]

I am a beginner, but I would like to work on this issue, can I?

[rnjudge]

@AyahRamahi Go for it! We always love help. Let us know if you have any questions.