[Code health] Throw exception instead of returning Error on relevant conversion functions

[gino-m]

Returning Error adds complexity at the call site, whereas exceptions move handling away from the call site. We should instead only return Error internally on functions which are intended to be called on multiple items (e.g., via map()) and filtered.

[gino-m]

@andreia-ferreira @gino-m For errors, we should throw an exception and let the caller handle it, but for recoverable warnings the question is more nuanced. Letting the caller log these would add a lot of complexity to the call stack; each layer would need to propagate a value or an error for each item, which would make method signatures and iterating over those errors complex.

For readability let's log recoverable warnings at the actually point where they happen rather than returning errors for now. If we need to propagate a signal up, we can add callback to report warnings in the future.