Provide error messaging around AAD mismatch detection

[piotr-roslaniec]

• ferveo API users have trouble debugging cases where CiphertextVerificationFailed is thrown in tpke
• This error is thrown by check_ciphertext_validity(ciphertext, aad, g_inv)
• Currently, we return ThresholdEncryptionError from the ferveo API, which is a catch-all for all tpke errors, including CiphertextVerificationFailed
• Since g_inv is invariant (higher-level ferveo API provided a hardcoded default), this error is only thrown when ciphertext and aad are mismatched
• Knowing this we can re-interpret the CiphertextVerificationFailed error and provide more concrete error messaging instead of ThresholdEncryptionError, for example: AADMismatchDuringCiphertextVerification

[arjunhassard]

Is this done?

[piotr-roslaniec]

@arjunhassard, no, but it's also not urgent IMHO