fix: Handle payment errors during user flow

[zachaysan]

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Changes

Ticket reference

Before this PR we were treating all errors from the chargebee client as one thing and we responded to a user that was in the process of signing up by issuing a 500 error, we were also logging all the chargebee errors as exceptions even though a declined credit card shouldn't be. Now, if the error is payment related, a 400 error gets sent to the client with a message to contact their administrator.

How did you test this code?

A combination of manual probing through the code with the test suite on and two new tests.

Remaining TODOs

• Figure out what other callsites there are that will be impacted by this change, if any.
• See if there are any other other areas of the codebase with invite-related code need to handle payment failures.
• Manually test this somehow. I have access to flagsmith-test.chargebee.com but I'm not sure what server it is connected to. Is that staging?
• [ ] Team up with @novakzaballa and get the frontend hooked up so the change will actually matter.

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Name	Status	Preview	Comments	Updated (UTC)
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[github-actions]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7d16197) 95.64% compared to head (9a65899) 95.65%.
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2951      +/-   ##
==========================================
+ Coverage   95.64%   95.65%   +0.01%     
==========================================
  Files        1012     1012              
  Lines       29173    29207      +34     
==========================================
+ Hits        27902    27938      +36     
+ Misses       1271     1269       -2     

Files	Coverage Δ
api/organisations/chargebee/chargebee.py	99.03% <100.00%> (+0.04%)	⬆️
api/organisations/invites/tests/test_views.py	100.00% <100.00%> (ø)
api/organisations/subscriptions/exceptions.py	100.00% <100.00%> (ø)
...sations/chargebee/test_unit_chargebee_chargebee.py	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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