Handle Felt252SerdeError

[Eagle941]

Method ContractClass::extract_sierra_program returns an error of type Felt252SerdeError. This error enum is private which prevents nice integration with thiserror crate and the error module.

At the moment the value returned by extract_sierra_program is just unwrapped which may lead to panic (though low probability because it doesn't depend on user inserted data).
Proper fix requires changes to the crate cairo-lang-starknet-classes.