Remove callback_exception

[odow]

There was actually a bug here: we passed cb_data in the try-catch, but cb in the invalid state check. So errors during the callback actually errored because .callback_data isn't a field. Rather than split the functions, I just made them in-line.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (405a031) to head (ea6622c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
- Coverage   98.61%   98.61%   -0.01%     
==========================================
  Files           3        3              
  Lines        2524     2523       -1     
==========================================
- Hits         2489     2488       -1     
  Misses         35       35              

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