Propagate exceptions thrown by custom RpcSmartSubtransport

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Background

If an exception is thrown in a custom RpcSmartSubtransport the exception is consumed by the SmartSubtransportStream.SetError code.

Related Fork PR: https://github.com/OctopusDeploy/libgit2sharp/pull/9

Result

This PR ensures that an exception thrown from within a RpcSmartSubtransport is passed to the Proxy.git_error_set_str call that is later re-retrieved and thrown to the consumer.