Use typed errors in user-facing APIs

[piotr-roslaniec]

• Closes Unroll error enum into anyhow stacktrace #90
• Closes Add serialization to DkgPublicKey bindings #89
• Consider adding custom Python Exceptions per enum Error variant
• Python traceback example:

Traceback (most recent call last):
  File "<...>/ferveo/ferveo-python/examples/server_api.py", line 108, in <module>
    decrypt_with_shared_secret(ciphertext, aad, shared_secret, dkg.g1_inv)
ValueError: Ciphertext verification failed

[KPrasch]

LGTM