cleanup(pubsub): future-proof AckError

[dbolduc]

Fixes #4809, related to #4508

We discussed whether the RPC branch should parse the string the server returns into an enum. My official stance on the matter is "why....".

We remove the details field for now, and mark the branch as non-exhaustive.

Non breaking, because AckError was never released.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.09%. Comparing base (0fc24aa) to head (ed6c8f0).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4979      +/-   ##
==========================================
+ Coverage   93.06%   93.09%   +0.02%     
==========================================
  Files         221      221              
  Lines        8611     8611              
==========================================
+ Hits         8014     8016       +2     
+ Misses        597      595       -2     

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