Return errors properly instead of just panicking

[Shaptic]

What

Prefer errors to panics.

Why

While these branches should be impossible to hit under normal circumstances (e.g. a failed unmarshaling would indicate that the Core node is returning invalid XDR), it is still good defensive programming.

Known limitations

n/a

[Copilot]

Pull request overview

This pull request improves error handling in the CGo interface by replacing panic calls with proper error logging and returning empty structs to signal errors to the Rust caller. This is a defensive programming improvement that allows errors to be gracefully handled instead of crashing the program when the Core node returns invalid XDR data.

Changes:

• Replaced two panic calls with error logging and empty struct returns in SnapshotSourceGet function
• Added error logging for SafeUnmarshal() and MarshalBinary() failures

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