[bug][flagd] Abort execution for custom operations, instead of just logging an error, and continuing with the evaluation

[aepfli]

The Python SDK only logs and error and continues its targeting evaluation.

The other SDKs, for example, Java or js are throwing an exception and don't continue the evaluation. This is also the specified flagd behavior.

We should align the implementation with the specified behaviour to ensure the same experience in all SDKs

relates: #91