Allow for blank OCW terms/years (and adjust readable_id accordingly)

[mbertrand]

What are the relevant tickets?

Closes #441

Description (What does it do?)

• Allows for blank terms/years in OCW courses, and adjusts the readable_id value accordingly
• If exceptions occur during a batch of ocw url paths processed by the learning_resources.etl.pipelines.ocw_courses_etl function, raise an ExtractException at the end indicating which ocw url's failed.

How can this be tested?

• Use the same AWS credentials and OCW_* settings as RC in your .env file.
• Run ./manage.py backpopulate_ocw_data --course-name=res-hs-002-chemistry-behind-the-magic-chemical-demonstrations-for-the-classroom
• Check that the learning resource imported, and the readable id is RES.HS-002
• Temporarily modify this line in learning_resources.etl.ocw.transform_run function to be the same as the main branch:

   "year": course_data.get("year")

• Restart containers and run the mgmt command again (with --overwrite). You should get an error.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (79891da) 77.01% compared to head (07fdd7b) 76.97%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #475      +/-   ##
==========================================
- Coverage   77.01%   76.97%   -0.04%     
==========================================
  Files         245      245              
  Lines        9998    10007       +9     
  Branches     1707     1708       +1     
==========================================
+ Hits         7700     7703       +3     
- Misses       2130     2135       +5     
- Partials      168      169       +1     

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[ChristopherChudzicki]

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