Fix #19858: Make MailChimp Error Handling Comprehensive

[U8NWXD]

Overview

1. This PR fixes [BUG]: Unable to Register on Oppia.org #19858.
2. This PR does the following: Ensures that all errors from MailChimp while signing a user up for the mailing list are caught, even those errors that arise when trying to handle missing users. Note that this fully resolves the [BUG]: Unable to Register on Oppia.org #19858 issue because it ensures that future configuration errors don't block user signups. We have already resolved the immediate problem by making the name fields not required in the MailChimp database.
3. (For bug-fixing PRs only) The original bug occurred because: The MailChimp server configuration was changed to require names, but the web app was not updated to provide those names. This caused user signups to the mailing list to fail, and poor error-handling led those failures to block user registration.

Essential Checklist

Please follow the instructions for making a code change.

• I have linked the issue that this PR fixes in the "Development" section of the sidebar.
• I have checked the "Files Changed" tab and confirmed that the changes are what I want to make.
• I have written tests for my code.
• The PR title starts with "Fix #bugnum: " or "Fix part of #bugnum: ...", followed by a short, clear summary of the changes.
• I have assigned the correct reviewers to this PR (or will leave a comment with the phrase "@{{reviewer_username}} PTAL" if I can't assign them directly).

Proof that changes are correct

This PR only changes how errors from MailChimp (which shouldn't ever happen and no longer happen on production) are handled. Correctness is confirmed by backend tests which use mocks to artificially induce MailChimp errors, but since these changes only activate under conditions that shouldn't ever happen, we can't test this end-to-end.

PR Pointers

• Never force push! If you do, your PR will be closed.
• To reply to reviewers, follow these instructions: https://github.com/oppia/oppia/wiki/Make-a-pull-request#step-5-address-review-comments-until-all-reviewers-approve
• Some e2e tests are flaky, and can fail for reasons unrelated to your PR. We are working on fixing this, but in the meantime, if you need to restart the tests, please check the "If your build fails" wiki page.
• See the Code Owner's wiki page for what code owners will expect.

[oppiabot]

Hi @U8NWXD, can you complete the following:

1. The proof that changes are correct has not been provided, please make sure to upload a image/video showing that the changes are correct. Or include a sentence saying "No proof of changes needed because" and the reason why proof of changes cannot be provided.
   Thanks!

[oppiabot]

Hi @vojtechjelinek, @DubeySandeep, @kevintab95, PTAL at this PR, it modifies files in jobs or platform folders.
Also @U8NWXD, please make sure to fill in the server jobs form for the new job or feature to be tested on the backup server. This PR can be merged only after the test run is successful. Please refer to this guide for details.
Thanks!

[oppiabot]

Hi @U8NWXD please assign the required reviewer(s) for this PR. Thanks!

[U8NWXD]

Per @seanlip this PR doesn't need to go through backup server testing because we can't test mailchimp on the backup server

[lkbhitesh07]

Thanks for the changes @U8NWXD, LGTM for the code owner files.

[oppiabot]

Hi @U8NWXD, this PR is ready to be merged. Please address any remaining comments prior to merging, and feel free to merge this PR once the CI checks pass and you're happy with it. Thanks!