Fix bug regarding serializing custom exceptions which occurs in multiprocessing context

[basnijholt]

Summary

The bug can be reproduced with:

from qiskit_ionq.exceptions import IonQAPIError
import pickle
err = IonQAPIError(message='hi', status_code=400, headers='hi', body='hi', error_type='hi')
pickle.loads(pickle.dumps(err))

which gives:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[5], line 4
      2 import pickle
      3 err = IonQAPIError(message='hi', status_code=400, headers='hi', body='hi', error_type='hi')
----> 4 pickle.loads(pickle.dumps(err))

TypeError: IonQAPIError.__init__() missing 4 required positional arguments: 'status_code', 'headers', 'body', and 'error_type'

[amritpoudelionq]

@basnijholt also specify OS?

[basnijholt]

@amritpoudelionq, the problem occurs on any OS or architecture.

[amritpoudelionq]

do we understand why __reduce__ works? __getstate__ and __setstate__ are pretty standard way for serialization?

[splch]

i just tested this out and it seems like __getstate__ and __setstate__ are relying on an instantiated object (which is why we see the error TypeError: IonQAPIError.__init__() missing 4 required positional arguments: 'status_code', 'headers', 'body', and 'error_type') but __reduce__ returns the class and arguments to instantiate it.